FA-75831 / Text diff and three-way merge / Open access

Diffstat path truncation: the ellipsis is not counted in the width · case 01

Truncated paths are three characters wider than the column.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The kept tail uses the full width and then the ellipsis is added.

VERIFIED REPAIR

Keep width - 3 characters so the ellipsis fits in the width.

Unsuccessful approach: Keeping width - 2 characters still overflows by one.

Case contract

A path longer than width is shown as "..." plus its last width-3 characters, further shortened to start at the first "/" inside that tail when there is one (keeping the slash). Paths not longer than width are unchanged.

Why this case matters

Narrow terminals truncate long paths in diffstat output; the rule keeps the most informative part of the path.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(names, width):
    res = []
    for name in names:
        if len(name) <= width:
            res.append(name)
            continue
        keep = width
        tail = name[len(name) - keep:] if keep > 0 else ''
        slash = tail.find('/')
        if slash >= 0:
            tail = tail[slash:]
        res.append('...' + tail)
    return res
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = {
    1: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    2: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    3: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    4: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    5: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
}[N]
for label, args, expected in cases:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
short name unchanged	`['src/a.c']`	`['src/a.c']`	Passed
name exactly the width	`['abcdefghij']`	`['abcdefghij']`	Passed
long name cut at a directory boundary	`['.../deep/file_name.py']`	`['.../file_name.py']`	Failed
no slash in the kept tail	`['...filename.txt']`	`['...ename.txt']`	Failed
several names	`['.../e/f/g/h.txt', 'x.c', '.../yyyyyyyyyyy']`	`['.../g/h.txt', 'x.c', '...yyyyyyyyy']`	Failed
tail starts exactly at a slash	`['.../bbbb/cc']`	`['.../cc']`	Failed
width just above the name	`['p/q/r.txt']`	`['p/q/r.txt']`	Passed

SHA-256 / a723e2da4f01e4a883842ea8b20e3734dda6a1803efe4fec31a8c029d4aac149

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(names, width):
    res = []
    for name in names:
        if len(name) <= width:
            res.append(name)
            continue
        keep = width - 2
        tail = name[len(name) - keep:] if keep > 0 else ''
        slash = tail.find('/')
        if slash >= 0:
            tail = tail[slash:]
        res.append('...' + tail)
    return res
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = {
    1: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    2: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    3: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    4: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    5: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
}[N]
for label, args, expected in cases:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
short name unchanged	`['src/a.c']`	`['src/a.c']`	Passed
name exactly the width	`['abcdefghij']`	`['abcdefghij']`	Passed
long name cut at a directory boundary	`['.../file_name.py']`	`['.../file_name.py']`	Passed
no slash in the kept tail	`['...lename.txt']`	`['...ename.txt']`	Failed
several names	`['.../f/g/h.txt', 'x.c', '...yyyyyyyyyy']`	`['.../g/h.txt', 'x.c', '...yyyyyyyyy']`	Failed
tail starts exactly at a slash	`['.../cc']`	`['.../cc']`	Passed
width just above the name	`['p/q/r.txt']`	`['p/q/r.txt']`	Passed

SHA-256 / ef9c0b036a4019d233a397bc950312d65b7f16427bf27feca1fdfc4fe343b83a

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(names, width):
    res = []
    for name in names:
        if len(name) <= width:
            res.append(name)
            continue
        keep = width - 3
        tail = name[len(name) - keep:] if keep > 0 else ''
        slash = tail.find('/')
        if slash >= 0:
            tail = tail[slash:]
        res.append('...' + tail)
    return res
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = {
    1: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    2: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    3: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    4: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
    5: [('short name unchanged', [['src/a.c'], 20], ['src/a.c']), ('name exactly the width', [['abcdefghij'], 10], ['abcdefghij']), ('long name cut at a directory boundary', [['src/module/deep/file_name.py'], 18], ['.../file_name.py']), ('no slash in the kept tail', [['averyveryverylongfilename.txt'], 12], ['...ename.txt']), ('several names', [['a/b/c/d/e/f/g/h.txt', 'x.c', 'dir/yyyyyyyyyyyyyyy'], 12], ['.../g/h.txt', 'x.c', '...yyyyyyyyy']), ('tail starts exactly at a slash', [['aaaa/bbbb/cc'], 8], ['.../cc']), ('width just above the name', [['p/q/r.txt'], 10], ['p/q/r.txt'])],
}[N]
for label, args, expected in cases:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
short name unchanged	`['src/a.c']`	`['src/a.c']`	Passed
name exactly the width	`['abcdefghij']`	`['abcdefghij']`	Passed
long name cut at a directory boundary	`['.../file_name.py']`	`['.../file_name.py']`	Passed
no slash in the kept tail	`['...ename.txt']`	`['...ename.txt']`	Passed
several names	`['.../g/h.txt', 'x.c', '...yyyyyyyyy']`	`['.../g/h.txt', 'x.c', '...yyyyyyyyy']`	Passed
tail starts exactly at a slash	`['.../cc']`	`['.../cc']`	Passed
width just above the name	`['p/q/r.txt']`	`['p/q/r.txt']`	Passed

SHA-256 / ff162986d34f41572b7f9289a68e9da9931d4ac6dd15db61df73d5a9e83f9967

Verification & scope

A deterministic, bounded teaching model of one diff, patch or merge rule with stipulated conventions; it is not a production diff or version-control implementation and makes no claim of conformance to any specific tool. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:49:10.415670+00:00.

Case digest / 24fcd0d4fa3801c2a192dcfd834825bd30f005a13e5308a94a4d6b98f6bda45f