Track testprogram/ again, add claudeCodeChatLog.md

The test pair verify.py runs against is back in the repo, so a fresh clone
can run the built-in checks. Conversion output (*-sab2sax-*) stays ignored,
so converting inside testprogram/ still leaves the tree clean.

claudeCodeChatLog.md records how this got built: what was asked for, the
decisions worth remembering (plain paths in system.properties, the port
source, Java float formatting, the f4-rounded manifest default, why a sab
cannot return runtime props) and what is still untested.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,4 @@
 __pycache__/
 *.pyc
-testprogram/
 *-sab2sax-*.sab
 *-sab2sax-*.sax

README.md

@@ -10,13 +10,46 @@ runtime lib).
 Verified byte-identical to `sedonac.exe` in both directions on a 189-component / 217-link app across
 18 kits.
 
-## Usage
+## Run it
 
-    python sab2sax.py [in.sax|in.sab] [out]
+    python run.py                    the window
+    pythonw run.py                   the window with no console behind it
+    python run.py app.sab [out]      straight to the command line converter
+
+`run.py` is the single entry point: no arguments opens the GUI, any argument is handed to the
+command line converter. Double-click it (or a shortcut to `pythonw run.py`) and you get the window.
+
+## The window
+
+    python run.py            (or: python gui.py)
+
+Title bar carries the name and version (`SAB - SAX Sedona Files Converter 0.0.0.002`). One button
+per direction — **SAB file to SAX file** and **SAX file to SAB file** — and the button you press
+decides which extension the file chooser offers. The log shows the chosen file and its folder
+*before* the conversion runs, then the output file and its folder when it finishes:
+
+    sab -> sax
+      INPUT FILE:
+        app.sab
+      INPUT PATH:
+        C:\apps
+      OUTPUT FILE:
+        app-sab2sax-20260728-140609.sax
+      OUTPUT PATH:
+        C:\apps
+      wrote 61294 bytes
+
+The result is written next to the input under that timestamped name, so nothing is overwritten and
+no save dialog gets in the way. Sedona home is shown at the top with a `Change...` button that
+rewrites `system.properties`.
+
+## The command line
+
+    python run.py [in.sax|in.sab] [out]      (or: python sab2sax.py ...)
 
 Like `sedonac`, the direction follows the input's extension: feed it a `.sax` and you get a `.sab`,
-feed it a `.sab` and you get a `.sax`. Both arguments are optional, so you can run it by
-double-click:
+feed it a `.sab` and you get a `.sax`. Both arguments are optional — `sab2sax.py` on its own asks for
+what it needs:
 
 * no input → a file chooser opens, listing `.sax` and `.sab`;
 * no output → a save dialog opens next to the input, pre-filled with a timestamped name:
@@ -63,36 +96,56 @@ set the key by hand instead.
 
 ## Version
 
-`config.NAME` / `config.VERSION` — currently `sab2sax 0.0.0.001`. Every dialog title bar reads
-`sab2sax 0.0.0.001 - Select the .sax or .sab application file to convert`, and the CLI prints the
-same line when it starts, so it is obvious which build produced a file. Bump it in
-[config.py](config.py) and both follow.
+`config.NAME` / `config.VERSION` — currently `SAB - SAX Sedona Files Converter 0.0.0.002`. The window
+title and every dialog title bar carry it; a chooser reads `SAB - SAX Sedona Files Converter
+0.0.0.002 - Select the .sax or .sab application file to convert`, and the CLI prints the same line
+when it starts, so it is obvious which build produced a file. Bump it in [config.py](config.py) and
+all of them follow.
 
 ## Verify
 
-    python verify.py [sedona_home]
+    python verify.py [app.sax|app.sab] [sedona_home]
 
-Without an argument the home comes from `system.properties`. Three checks, run against a
-`test_normal.sax` / `test_normal_sedonac.sab` pair in `testprogram/`. That folder is **not** in the
-repo (gitignored) — drop the pair in yourself, otherwise verify prints `SKIP` and exits 0:
+Point it at any app of yours and it round-trips it, direction by extension:
 
-    test_normal.sax -> sab        vs test_normal_sedonac.sab
-    test_normal_sedonac.sab -> sax  vs test_normal.sax
-    test_normal_sedonac.sab -> sax -> sab   back to the same bytes
+    app.sab  ->  sab -> sax -> sab      must come back byte-identical
+    app.sax  ->  sax -> sab -> sax -> sab   the two sabs must be identical
+
+The `.sax` case compares the two **binaries**, not the two texts, because going through a `.sab` is
+lossy on purpose — runtime prop values and formatting are not in the binary. If the regenerated
+`.sax` differs from the original, that is printed as a note rather than a failure. (For a `.sax`
+sedonac itself wrote, they do match.)
+
+With no file argument a chooser opens, listing `.sax` and `.sab`. Cancel it and the built-in
+regression runs instead, against the `test_normal.sax` / `test_normal_sedonac.sab` pair checked in
+under `testprogram/`. Remove them and verify prints `SKIP` and exits 0 instead of failing:
+
+    test_normal.sax -> sab            vs test_normal_sedonac.sab
+    test_normal_sedonac.sab -> sax    vs test_normal.sax
+    both round trips as above
 
 `check_encode(sax)` / `check_decode(sab)` with no reference argument run `bin/sedonac.exe` on a copy
 of the input and diff against that instead — a live comparison, if you have a JRE it will start
 under.
 
+Exit code is 0 on pass, 1 on failure. A malformed file fails with a one-line reason
+(`comp 0 app: missing child 254`) rather than a traceback.
+
 ## Files
 
+    run.py               entry point: window with no arguments, CLI with them
+    gui.py               the window: a button per direction
     sab2sax.py           the CLI: dispatches on the input extension
     sab.py               sax -> sab encoder
     sax.py               sab -> sax decoder
     config.py            system.properties read/write, folder + file choosers
     verify.py            regression check against sedonac's output, both directions
     system.properties    sedona.home
-    testprogram/         local only, gitignored: test apps to run verify.py against
+    testprogram/         test apps verify.py runs against
+    claudeCodeChatLog.md how this got built, and why
+
+Conversion output (`*-sab2sax-*.sax`, `*-sab2sax-*.sab`) is gitignored, so converting inside
+`testprogram/` leaves the repo clean.
 
 ## Format
 

claudeCodeChatLog.md

@@ -0,0 +1,86 @@
+# Claude Code session log
+
+What was asked for, what was built, and the decisions behind it. Written 2026-08-13, covering the
+work that produced `ec4019e` .. `2dcc170` (tags `v0.0.0.001`, `v0.0.0.002`).
+
+Starting point: `sab.py`, a pure-Python `.sax` -> `.sab` encoder taking `<app.sax> <sedona_home>
+<out.sab>` on the command line, plus `verify.py`.
+
+## What was asked, in order
+
+1. **Sedona home into `system.properties`**, with a folder chooser on first run, and a file chooser
+   for the input.
+2. **Plain paths, not java-escaped**, in that properties file.
+3. **Timestamped output names** — `[name]-sab2sax-20260728-140609`.
+4. **The reverse direction**, `.sab` -> `.sax`, direction chosen by the input's extension.
+5. **A version**, shown in dialog titles.
+6. **`testprogram/` out of the repo** — later reversed: the test pair is checked in again, while
+   conversion output (`*-sab2sax-*`) stays gitignored.
+7. **A file chooser in `verify.py`** and a round-trip check on whatever is chosen.
+8. **A window** with a button per direction, showing the chosen file before converting and the
+   output afterwards.
+9. **`run.py`** as the entry point; log labels in capitals, values on their own lines.
+
+## What exists now
+
+    run.py               entry point: window with no arguments, CLI with them
+    gui.py               the window: a button per direction
+    sab2sax.py           the CLI: dispatches on the input extension
+    sab.py               sax -> sab encoder
+    sax.py               sab -> sax decoder
+    config.py            system.properties, dialogs, NAME/VERSION
+    verify.py            round trips + comparison against sedonac output
+    system.properties    sedona.home
+    testprogram/         test apps verify.py runs against
+
+`sedona.home` lives in `system.properties`. Missing or pointing somewhere without `manifests/` and a
+folder chooser opens, then the answer is written back. Output files are always written next to the
+input as `<stem>-sab2sax-<YYYYMMDD-HHMMSS>.<ext>`, so a run never overwrites the previous one.
+
+## Decisions worth remembering
+
+**Plain paths in `system.properties`.** Java's `Properties` (what sedonac reads it with) treats `\`
+as an escape and doubles them up; we only read the file ourselves, so we write plain Windows paths.
+The parser collapses `\\` to `\` and keeps every other backslash literal — both forms load, and
+`C:\niagara` never becomes a newline.
+
+**The decoder is a port, not a guess.** `sax.py` follows `OfflineApp.decodeAppBinary` +
+`encodeAppXml` and `OfflineComponent`/`OfflineLink` from
+`sedonaSrcOrginal/sedona/src/sedona/offline/`. The tree is rebuilt from the
+`firstChild`/`nextSibling` id chain; the XML is written the way `XWriter` does it — two-space
+indent, a `<!-- /path -->` comment above every `<comp>`, `&#x..;` escaping.
+
+**Java float formatting.** `Value.encodeString` for a float is `java.lang.Float.toString`: shortest
+digits that round-trip, always a decimal point, `E` form outside `[1e-3, 1e7)`. `sax.java_float`
+reproduces it. It matches modern Java rather than the old JDK's extra-digit quirk on subnormals
+(`1.4E-45` vs our `1.0E-45`) — only reachable with denormal floats.
+
+**The manifest default is a 32-bit float too.** A prop equal to its default is not written to the
+`.sax`. Comparing a decoded `float` against a default parsed as a Python double makes `0.1` look
+different from `0.1`, and every `minDelta="0.1"` gets written where sedonac omits it. Rounding the
+default to `f4` first was the last diff between "close" and "identical".
+
+**A `.sab` cannot give back runtime props.** Only config props are in the binary. sedonac has the
+same hole. That is why a `.sax` round trip compares the two *binaries*, not the two texts.
+
+## Verification
+
+- `test_normal.sax` -> sab, byte-identical to `test_normal_sedonac.sab` (7852 bytes)
+- `test_normal_sedonac.sab` -> sax, byte-identical to `test_normal.sax` (61294 bytes)
+- both round trips clean: `sab -> sax -> sab`, and `sax -> sab -> sax -> sab`
+
+A live `sedonac.exe` comparison was attempted and could not run here — it fails with `Cannot load
+library: C:\Program Files\Zulu\zulu-21-jre\bin\server\jvm.dll`. `verify.check_encode(sax)` /
+`check_decode(sab)` with no reference argument still do that comparison on a machine with a JRE
+sedonac will start under.
+
+Malformed input fails with one line and exit 1, not a traceback: `comp 0 app: missing child 254`.
+
+## Where to pick up
+
+- Untested paths are unchanged from before: action overrides, `Buf`-typed props, non-ASCII strings,
+  and apps whose ids exceed the 256-entry lookup table.
+- `verify.py` prints `SKIP` and exits 0 when `testprogram/` is empty rather than failing; the test
+  pair is checked in, so it normally runs.
+- Version lives in `config.py` (`NAME`, `VERSION`). The window title, every dialog title and the CLI
+  banner all read from it.

config.py

@@ -14,8 +14,8 @@ chooser is shown and the answer is written back to `system.properties`.
 """
 import os, sys
 
-NAME = "sab2sax"
-VERSION = "0.0.0.001"
+NAME = "SAB - SAX Sedona Files Converter"
+VERSION = "0.0.0.002"
 
 HERE = os.path.dirname(os.path.abspath(__file__))
 PROPS = os.path.join(HERE, "system.properties")
@@ -169,7 +169,15 @@ def choose_input_file(initial=None):
 
 
 def choose_sax_file(initial=None):
-    return ask_open_file("Select the .sax application file", [SAX, ANY], initial)
+    """Open a .sax -- the input of a sax -> sab conversion."""
+    return ask_open_file("Select the .sax application file to convert to .sab",
+                         [SAX, ANY], initial)
+
+
+def choose_sab_input(initial=None):
+    """Open a .sab -- the input of a sab -> sax conversion."""
+    return ask_open_file("Select the .sab application file to convert to .sax",
+                         [SAB, ANY], initial)
 
 
 def choose_sab_file(initialfile=None, initial=None):

gui.py

@@ -0,0 +1,166 @@
+"""A window with two buttons, one per direction.
+
+    python gui.py
+
+Pick a direction, pick a file; the log shows what was chosen before the
+conversion runs and where the result landed afterwards. Output goes next to the
+input under a timestamped name, so nothing is ever overwritten.
+"""
+import os, sys, traceback
+import tkinter as tk
+from tkinter import ttk, messagebox
+
+import config, sab, sax
+
+
+class App:
+    def __init__(self, root):
+        self.root = root
+        root.title("%s %s" % (config.NAME, config.VERSION))
+        root.minsize(620, 340)
+
+        frame = ttk.Frame(root, padding=12)
+        frame.pack(fill="both", expand=True)
+        frame.columnconfigure(0, weight=1)
+        frame.rowconfigure(3, weight=1)
+
+        # sedona home
+        home_box = ttk.LabelFrame(frame, text="Sedona home", padding=(8, 4))
+        home_box.grid(row=0, column=0, sticky="ew")
+        home_box.columnconfigure(0, weight=1)
+        self.home_var = tk.StringVar(value="(not set)")
+        ttk.Label(home_box, textvariable=self.home_var).grid(row=0, column=0, sticky="w")
+        ttk.Button(home_box, text="Change...", command=self.change_home).grid(row=0, column=1, padx=(8, 0))
+
+        # the two directions
+        buttons = ttk.Frame(frame, padding=(0, 12))
+        buttons.grid(row=1, column=0, sticky="ew")
+        buttons.columnconfigure(0, weight=1)
+        buttons.columnconfigure(1, weight=1)
+        self.b_sab2sax = ttk.Button(buttons, text="SAB file to SAX file", padding=10,
+                                    command=lambda: self.run("sab"))
+        self.b_sab2sax.grid(row=0, column=0, sticky="ew", padx=(0, 6))
+        self.b_sax2sab = ttk.Button(buttons, text="SAX file to SAB file", padding=10,
+                                    command=lambda: self.run("sax"))
+        self.b_sax2sab.grid(row=0, column=1, sticky="ew", padx=(6, 0))
+
+        self.status = tk.StringVar(value="Choose a direction.")
+        ttk.Label(frame, textvariable=self.status).grid(row=2, column=0, sticky="w")
+
+        # log
+        log_box = ttk.Frame(frame)
+        log_box.grid(row=3, column=0, sticky="nsew", pady=(8, 0))
+        log_box.columnconfigure(0, weight=1)
+        log_box.rowconfigure(0, weight=1)
+        self.log_text = tk.Text(log_box, height=10, wrap="word", state="disabled")
+        self.log_text.grid(row=0, column=0, sticky="nsew")
+        bar = ttk.Scrollbar(log_box, orient="vertical", command=self.log_text.yview)
+        bar.grid(row=0, column=1, sticky="ns")
+        self.log_text.configure(yscrollcommand=bar.set)
+
+        self.home = None
+        root.after(50, self.first_home)
+
+    # ---------- helpers ----------
+    def log(self, line=""):
+        self.log_text.configure(state="normal")
+        self.log_text.insert("end", line + "\n")
+        self.log_text.see("end")
+        self.log_text.configure(state="disabled")
+        self.root.update_idletasks()      # so it is on screen before the work starts
+
+    def set_home(self, path):
+        self.home = path
+        self.home_var.set(path or "(not set)")
+
+    def first_home(self):
+        """Resolve sedona home at startup; the folder chooser opens if it isn't set."""
+        try:
+            self.set_home(config.get_sedona_home())
+            self.log("sedona home: %s" % self.home)
+        except SystemExit as e:
+            self.log("no sedona home: %s" % e)
+            messagebox.showwarning(config.title("Sedona home"),
+                                   "No sedona home selected.\n\n"
+                                   "Set it with Change... before converting.")
+
+    def change_home(self):
+        picked = config.ask_directory("Select Sedona home (the folder containing manifests/)",
+                                      self.home)
+        if not picked:
+            return
+        picked = os.path.normpath(picked)
+        if not config.is_sedona_home(picked):
+            messagebox.showerror(config.title("Sedona home"),
+                                 "%s has no manifests/ subfolder." % picked)
+            return
+        config.write_prop(config.KEY, picked)
+        self.set_home(picked)
+        self.log("sedona home: %s  (saved to %s)" % (picked, config.PROPS))
+
+    # ---------- conversion ----------
+    def run(self, direction):
+        """direction: 'sab' means sab -> sax, 'sax' means sax -> sab."""
+        if not config.is_sedona_home(self.home):
+            messagebox.showerror(config.title("Sedona home"),
+                                 "Set a valid sedona home first (Change...).")
+            return
+
+        src = config.choose_sab_input() if direction == "sab" else config.choose_sax_file()
+        if not src:
+            self.status.set("Cancelled.")
+            return
+        src = os.path.abspath(src)
+
+        # show what was chosen, before doing anything with it
+        self.log()
+        self.log("%s -> %s" % (direction, "sax" if direction == "sab" else "sab"))
+        self.log("  INPUT FILE:")
+        self.log("    %s" % os.path.basename(src))
+        self.log("  INPUT PATH:")
+        self.log("    %s" % os.path.dirname(src))
+        self.status.set("Converting %s ..." % os.path.basename(src))
+
+        if direction == "sab":
+            out = os.path.join(os.path.dirname(src), sax.default_sax_name(src))
+            work = lambda: sax.decode(src, self.home, out)
+        else:
+            out = os.path.join(os.path.dirname(src), sab.default_sab_name(src))
+            work = lambda: sab.encode(src, self.home, out)
+
+        self.busy(True)
+        try:
+            n = work()
+        except Exception as e:
+            self.busy(False)
+            self.log("  FAILED: %s" % e)
+            self.status.set("Failed.")
+            traceback.print_exc()
+            messagebox.showerror(config.title("Conversion failed"), str(e))
+            return
+        self.busy(False)
+
+        self.log("  OUTPUT FILE:")
+        self.log("    %s" % os.path.basename(out))
+        self.log("  OUTPUT PATH:")
+        self.log("    %s" % os.path.dirname(out))
+        self.log("  wrote %d bytes" % n)
+        self.status.set("Done: %s" % os.path.basename(out))
+
+    def busy(self, on):
+        state = "disabled" if on else "normal"
+        self.b_sab2sax.configure(state=state)
+        self.b_sax2sab.configure(state=state)
+        self.root.configure(cursor="watch" if on else "")
+        self.root.update_idletasks()
+
+
+def main():
+    root = tk.Tk()
+    App(root)
+    root.mainloop()
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

run.py

@@ -0,0 +1,22 @@
+"""Start here.
+
+    python run.py                  the window
+    pythonw run.py                 the window, without a console behind it
+    python run.py app.sab [out]    straight to the command line converter
+
+No arguments means the GUI; any argument is passed through to sab2sax.py, so
+the same entry point serves a double-click and a script.
+"""
+import sys
+
+
+def main(argv):
+    if len(argv) > 1:
+        import sab2sax
+        return sab2sax.main(argv) or 0
+    import gui
+    return gui.main()
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))

sax.py

@@ -225,15 +225,20 @@ def read_app(sab_path, home):
         order.append(cid)
 
     # rebuild the tree from the child/sibling id chain
+    def must(cid, what, owner):
+        if cid not in by_id:
+            raise Exception("comp %d %s: missing %s %d" % (owner["id"], owner["name"], what, cid))
+        return by_id[cid]
+
     for c in by_id.values():
-        c["parent"] = by_id[c["parentId"]] if c["parentId"] != 0xFFFF else None
+        c["parent"] = must(c["parentId"], "parent", c) if c["parentId"] != 0xFFFF else None
         if c["childId"] != 0xFFFF:
-            kid = by_id[c["childId"]]
+            kid = must(c["childId"], "child", c)
             while True:
                 c["kids"].append(kid)
                 if kid["siblingId"] == 0xFFFF:
                     break
-                kid = by_id[kid["siblingId"]]
+                kid = must(kid["siblingId"], "nextSibling", kid)
 
     links = []
     while True:

verify.py

@@ -1,27 +1,38 @@
-"""Regression check: our output must be byte-identical to sedonac.exe's, both ways.
+"""Regression checks: our output must be byte-identical to sedonac.exe's, and a
+conversion must survive a round trip.
 
-    sax -> sab   check_encode()
-    sab -> sax   check_decode()
-    sab -> sax -> sab  round trip
+    sax -> sab              check_encode()
+    sab -> sax              check_decode()
+    sab -> sax -> sab       round_trip_sab()   bytes must come back identical
+    sax -> sab -> sax -> sab round_trip_sax()  the two sabs must be identical
 
-Usage:  python verify.py [sedona_home]
+Usage:  python verify.py [app.sax|app.sab] [sedona_home]
 
-Without an argument the home comes from system.properties (see config.py), which
-pops a folder chooser the first time.
+With no file argument a chooser opens; cancel it and the built-in
+`testprogram/` regression runs instead (if those files are present). Sedona home
+comes from system.properties (see config.py) unless given.
 """
 import os, sys, subprocess, tempfile, shutil
 import sab, sax, config
 
 HERE = os.path.dirname(os.path.abspath(__file__))
-HOME = sys.argv[1] if len(sys.argv) > 1 else config.get_sedona_home()
 TESTS = os.path.join(HERE, "testprogram")
+HOME = None
+
+
+def home():
+    """Resolved once, on first use -- so importing this module opens no dialog."""
+    global HOME
+    if HOME is None:
+        HOME = config.get_sedona_home()
+    return HOME
 
 
 def sedonac(src, tmp):
     """Run sedonac on a copy of `src`; return the file it produced (needs a JRE)."""
     work = os.path.join(tmp, os.path.basename(src))
     shutil.copy(src, work)
-    exe = os.path.join(HOME, "bin", "sedonac.exe")
+    exe = os.path.join(home(), "bin", "sedonac.exe")
     r = subprocess.run([exe, work], capture_output=True, text=True)
     if r.returncode != 0:
         print("  sedonac failed:\n" + r.stdout + r.stderr)
@@ -30,30 +41,32 @@ def sedonac(src, tmp):
     return os.path.splitext(work)[0] + other
 
 
-def compare(label, reference, mine, binary):
+def compare(label, reference, mine, binary, names=("sedonac", "ours")):
     a = open(reference, "rb").read()
     b = open(mine, "rb").read()
     if a == b:
-        print("  PASS  %-28s %d bytes identical" % (label, len(a)))
+        print("  PASS  %-34s %d bytes identical" % (label, len(a)))
         return True
 
     diffs = [i for i in range(min(len(a), len(b))) if a[i] != b[i]]
-    print("  FAIL  %-28s sedonac=%d ours=%d, %d byte diffs" % (label, len(a), len(b), len(diffs)))
+    print("  FAIL  %-34s %s=%d %s=%d, %d byte diffs" % (
+        label, names[0], len(a), names[1], len(b), len(diffs)))
     if diffs:
         i = diffs[0]
         if binary:
             lo, hi = max(0, i - 12), i + 12
             print("        first diff at offset %d" % i)
-            print("        sedonac  %s" % a[lo:hi].hex())
-            print("        ours     %s" % b[lo:hi].hex())
+            print("        %-8s %s" % (names[0], a[lo:hi].hex()))
+            print("        %-8s %s" % (names[1], b[lo:hi].hex()))
         else:
             line = a[:i].count(b"\n") + 1
             print("        first diff at line %d" % line)
-            print("        sedonac  %s" % a.splitlines()[line - 1])
-            print("        ours     %s" % b.splitlines()[line - 1])
+            print("        %-8s %s" % (names[0], a.splitlines()[line - 1]))
+            print("        %-8s %s" % (names[1], b.splitlines()[line - 1]))
     return False
 
 
+# ---------- against sedonac ----------
 def check_encode(sax_file, reference=None):
     """sax -> sab, against `reference` or a fresh sedonac run."""
     tmp = tempfile.mkdtemp()
@@ -62,7 +75,7 @@ def check_encode(sax_file, reference=None):
         if reference is None:
             return False
         mine = os.path.join(tmp, "mine.sab")
-        sab.encode(sax_file, HOME, mine)
+        sab.encode(sax_file, home(), mine)
         return compare(os.path.basename(sax_file) + " -> sab", reference, mine, True)
     finally:
         shutil.rmtree(tmp, ignore_errors=True)
@@ -76,36 +89,108 @@ def check_decode(sab_file, reference=None):
         if reference is None:
             return False
         mine = os.path.join(tmp, "mine.sax")
-        sax.decode(sab_file, HOME, mine)
+        sax.decode(sab_file, home(), mine)
         return compare(os.path.basename(sab_file) + " -> sax", reference, mine, False)
     finally:
         shutil.rmtree(tmp, ignore_errors=True)
 
 
-def check_round_trip(sab_file):
+# ---------- round trips ----------
+def round_trip_sab(sab_file):
     """sab -> sax -> sab must come back to the same bytes."""
     tmp = tempfile.mkdtemp()
     try:
         mid = os.path.join(tmp, "mid.sax")
         back = os.path.join(tmp, "back.sab")
-        sax.decode(sab_file, HOME, mid)
-        sab.encode(mid, HOME, back)
-        return compare(os.path.basename(sab_file) + " round trip", sab_file, back, True)
+        sax.decode(sab_file, home(), mid)
+        sab.encode(mid, home(), back)
+        return compare(os.path.basename(sab_file) + " sab->sax->sab",
+                       sab_file, back, True, ("original", "ours"))
     finally:
         shutil.rmtree(tmp, ignore_errors=True)
 
 
-if __name__ == "__main__":
-    print("sedona home: %s" % HOME)
+def round_trip_sax(sax_file):
+    """sax -> sab -> sax -> sab: the two sabs must match.
+
+    The sax leg is not compared against the original file, because a .sab holds
+    no runtime prop values and no formatting -- coming back through it is lossy
+    on purpose. The binary is the thing that has to be stable, so we compare
+    that. Any difference from the original sax is printed as a note, not a
+    failure."""
+    tmp = tempfile.mkdtemp()
+    try:
+        first = os.path.join(tmp, "first.sab")
+        mid = os.path.join(tmp, "mid.sax")
+        second = os.path.join(tmp, "second.sab")
+        sab.encode(sax_file, home(), first)
+        sax.decode(first, home(), mid)
+        sab.encode(mid, home(), second)
+
+        ok = compare(os.path.basename(sax_file) + " sax->sab->sax->sab",
+                     first, second, True, ("1st sab", "2nd sab"))
+
+        a = open(sax_file, "rb").read()
+        b = open(mid, "rb").read()
+        if a != b:
+            al, bl = a.splitlines(), b.splitlines()
+            print("        note: the regenerated sax differs from the original "
+                  "(%d vs %d lines) -- expected where the original carries runtime "
+                  "props or different formatting" % (len(al), len(bl)))
+        return ok
+    finally:
+        shutil.rmtree(tmp, ignore_errors=True)
+
+
+def check_file(path):
+    """Round-trip whatever was chosen, by extension."""
+    ext = os.path.splitext(path)[1].lower()
+    if ext not in (".sax", ".sab"):
+        print("  don't know what to do with %s -- expected .sax or .sab" % path)
+        return False
+    try:
+        return round_trip_sax(path) if ext == ".sax" else round_trip_sab(path)
+    except Exception as e:      # a malformed file is a failure, not a traceback
+        print("  FAIL  %-34s %s" % (os.path.basename(path), e))
+        return False
+
+
+def builtin():
+    """The checked-in pair, when testprogram/ is populated locally."""
     saxf = os.path.join(TESTS, "test_normal.sax")
     sabf = os.path.join(TESTS, "test_normal_sedonac.sab")
     missing = [f for f in (saxf, sabf) if not os.path.isfile(f)]
     if missing:
         # testprogram/ is deliberately not in the repo -- put the pair back to run this
         print("  SKIP  no test files: %s" % ", ".join(os.path.basename(f) for f in missing))
-        sys.exit(0)
+        return None
 
     ok = check_encode(saxf, sabf)
     ok &= check_decode(sabf, saxf)
-    ok &= check_round_trip(sabf)
-    sys.exit(0 if ok else 1)
+    ok &= round_trip_sab(sabf)
+    ok &= round_trip_sax(saxf)
+    return ok
+
+
+def main(argv):
+    global HOME
+    print("%s %s" % (config.NAME, config.VERSION))
+    path = argv[1] if len(argv) > 1 else config.choose_input_file()
+    if len(argv) > 2:
+        HOME = argv[2]
+    print("sedona home: %s" % home())
+
+    if not path:
+        print("no file chosen -- running the built-in checks")
+        ok = builtin()
+        return 0 if ok is not False else 1
+
+    path = os.path.abspath(path)
+    if not os.path.isfile(path):
+        print("no such file: " + path)
+        return 1
+    return 0 if check_file(path) else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))