FA-80686 / Bidirectional text layout / Open access

Bidi hit testing: left clamp · case 01

Clicks left of the line hit characters at the far end.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Negative slots are not clamped, so they index from the end of the order.

VERIFIED REPAIR

Clamp the slot to at least zero.

Unsuccessful approach: Clamping to one skips the first slot.

Case contract

Input [levels, x in half-character units]. Clamp to a visual slot (each character 2 units). The right half of an LTR character or the left half of an RTL character is its trailing side. Points before the line use the left half of slot 0, beyond it the right half of the last slot. Return [logical index, trailing, caret offset].

Why this case matters

Mixed right-to-left and left-to-right text must resolve levels and visual order exactly, or words, numbers and carets land in the wrong place.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(x):
    def reorder(lv):
        vis = list(range(len(lv)))
        odd = [l for l in lv if l % 2 == 1]
        if not odd:
            return vis
        for lev in range(max(lv), min(odd) - 1, -1):
            i = 0
            while i < len(vis):
                if lv[vis[i]] >= lev:
                    j = i
                    while j < len(vis) and lv[vis[j]] >= lev:
                        j += 1
                    vis[i:j] = vis[i:j][::-1]
                    i = j
                else:
                    i += 1
        return vis
    
    levels, px = x
    n = len(levels)
    if n == 0:
        return [None, False, 0]
    vis = reorder(levels)
    v = min(px // 2, n - 1)
    i = vis[v]
    right_half = px - 2 * v >= 1
    rtl = levels[i] % 2 == 1
    trailing = right_half != rtl
    return [i, trailing, i + 1 if trailing else i]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[1, 3], -1], [1, True, 2]), ('partial-repair probe', [[1, 1, 1, 1, 1, 1, 1, 2, 2], 1], [7, True, 8]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1, 4, 2], -3], [6, False, 6]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[1, 1, 1, 1, 1, 1, 1, 0, 0], 15], [7, True, 8]), ('control layout', [[3, 2, 2, 3, 1, 1, 1, 1, 1], 10], [0, True, 1])], [('regression: left clamp', [[1, 1, 1, 1], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 1, 1, 3], -1], [5, True, 6]), ('regression: left clamp', [[1, 1, 1, 1, 0, 3, 3], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 0, 0, 0, 0], -2], [2, True, 3]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 1, 1, 1], 8], [1, False, 1]), ('control layout', [[2, 2, 2, 1, 1, 1, 1], 15], [2, True, 3])], [('regression: left clamp', [[3, 3, 3, 3, 1, 1, 1], -2], [6, True, 7]), ('regression: left clamp', [[2, 2, 2, 2, 2, 2], -1], [0, False, 0]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('partial-repair probe', [[1, 1, 1, 1], 1], [3, False, 3]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 2, 4, 4, 4], 8], [4, False, 4]), ('control layout', [[0, 0, 0, 3], 7], [3, False, 3])], [('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1], -1], [5, True, 6]), ('regression: left clamp', [[2, 2, 4, 4, 4, 4, 4, 2, 2], -1], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 2, 2, 2, 2, 0, 1, 1], 20], [8, False, 8]), ('control layout', [[2, 2, 2, 2], 7], [3, True, 4])], [('regression: left clamp', [[0, 0, 1, 1, 0], -1], [0, False, 0]), ('regression: left clamp', [[1, 1, 1, 1, 1, 3, 1, 1], -2], [7, True, 8]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('regression: left clamp', [[0, 1], -2], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 3, 3, 1, 2, 2, 2, 2], 11], [4, False, 4]), ('control layout', [[0, 0, 0, 1, 1, 1, 1, 3, 3], 3], [1, True, 2])]]
for label, args, expected in fixtures[N - 1]:
    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
click left of RTL line	`[0, False, 0]`	`[2, True, 3]`	Failed
regression: left clamp	`[0, False, 0]`	`[1, True, 2]`	Failed
partial-repair probe	`[7, True, 8]`	`[7, True, 8]`	Passed
regression: left clamp	`[1, False, 1]`	`[6, False, 6]`	Failed
right half of RTL char	`[1, False, 1]`	`[1, False, 1]`	Passed
click past end	`[1, False, 1]`	`[1, False, 1]`	Passed
control layout	`[7, True, 8]`	`[7, True, 8]`	Passed
control layout	`[0, True, 1]`	`[0, True, 1]`	Passed

SHA-256 / cb1feae39fb3ba513d19c20d8fa4dbae910b582d2a9c116a2f4913639ebf395b

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x):
    def reorder(lv):
        vis = list(range(len(lv)))
        odd = [l for l in lv if l % 2 == 1]
        if not odd:
            return vis
        for lev in range(max(lv), min(odd) - 1, -1):
            i = 0
            while i < len(vis):
                if lv[vis[i]] >= lev:
                    j = i
                    while j < len(vis) and lv[vis[j]] >= lev:
                        j += 1
                    vis[i:j] = vis[i:j][::-1]
                    i = j
                else:
                    i += 1
        return vis
    
    levels, px = x
    n = len(levels)
    if n == 0:
        return [None, False, 0]
    vis = reorder(levels)
    v = min(max(px // 2, 1), n - 1)
    i = vis[v]
    right_half = px - 2 * v >= 1
    rtl = levels[i] % 2 == 1
    trailing = right_half != rtl
    return [i, trailing, i + 1 if trailing else i]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[1, 3], -1], [1, True, 2]), ('partial-repair probe', [[1, 1, 1, 1, 1, 1, 1, 2, 2], 1], [7, True, 8]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1, 4, 2], -3], [6, False, 6]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[1, 1, 1, 1, 1, 1, 1, 0, 0], 15], [7, True, 8]), ('control layout', [[3, 2, 2, 3, 1, 1, 1, 1, 1], 10], [0, True, 1])], [('regression: left clamp', [[1, 1, 1, 1], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 1, 1, 3], -1], [5, True, 6]), ('regression: left clamp', [[1, 1, 1, 1, 0, 3, 3], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 0, 0, 0, 0], -2], [2, True, 3]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 1, 1, 1], 8], [1, False, 1]), ('control layout', [[2, 2, 2, 1, 1, 1, 1], 15], [2, True, 3])], [('regression: left clamp', [[3, 3, 3, 3, 1, 1, 1], -2], [6, True, 7]), ('regression: left clamp', [[2, 2, 2, 2, 2, 2], -1], [0, False, 0]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('partial-repair probe', [[1, 1, 1, 1], 1], [3, False, 3]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 2, 4, 4, 4], 8], [4, False, 4]), ('control layout', [[0, 0, 0, 3], 7], [3, False, 3])], [('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1], -1], [5, True, 6]), ('regression: left clamp', [[2, 2, 4, 4, 4, 4, 4, 2, 2], -1], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 2, 2, 2, 2, 0, 1, 1], 20], [8, False, 8]), ('control layout', [[2, 2, 2, 2], 7], [3, True, 4])], [('regression: left clamp', [[0, 0, 1, 1, 0], -1], [0, False, 0]), ('regression: left clamp', [[1, 1, 1, 1, 1, 3, 1, 1], -2], [7, True, 8]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('regression: left clamp', [[0, 1], -2], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 3, 3, 1, 2, 2, 2, 2], 11], [4, False, 4]), ('control layout', [[0, 0, 0, 1, 1, 1, 1, 3, 3], 3], [1, True, 2])]]
for label, args, expected in fixtures[N - 1]:
    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
click left of RTL line	`[1, True, 2]`	`[2, True, 3]`	Failed
regression: left clamp	`[0, True, 1]`	`[1, True, 2]`	Failed
partial-repair probe	`[8, False, 8]`	`[7, True, 8]`	Failed
regression: left clamp	`[7, False, 7]`	`[6, False, 6]`	Failed
right half of RTL char	`[0, True, 1]`	`[1, False, 1]`	Failed
click past end	`[1, False, 1]`	`[1, False, 1]`	Passed
control layout	`[7, True, 8]`	`[7, True, 8]`	Passed
control layout	`[0, True, 1]`	`[0, True, 1]`	Passed

SHA-256 / a4b895ba8d6a8656ad498efec4e26e70def60230123e320d02b465f78b8bac7d

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(x):
    def reorder(lv):
        vis = list(range(len(lv)))
        odd = [l for l in lv if l % 2 == 1]
        if not odd:
            return vis
        for lev in range(max(lv), min(odd) - 1, -1):
            i = 0
            while i < len(vis):
                if lv[vis[i]] >= lev:
                    j = i
                    while j < len(vis) and lv[vis[j]] >= lev:
                        j += 1
                    vis[i:j] = vis[i:j][::-1]
                    i = j
                else:
                    i += 1
        return vis
    
    levels, px = x
    n = len(levels)
    if n == 0:
        return [None, False, 0]
    vis = reorder(levels)
    v = min(max(px // 2, 0), n - 1)
    i = vis[v]
    right_half = px - 2 * v >= 1
    rtl = levels[i] % 2 == 1
    trailing = right_half != rtl
    return [i, trailing, i + 1 if trailing else i]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[1, 3], -1], [1, True, 2]), ('partial-repair probe', [[1, 1, 1, 1, 1, 1, 1, 2, 2], 1], [7, True, 8]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1, 4, 2], -3], [6, False, 6]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[1, 1, 1, 1, 1, 1, 1, 0, 0], 15], [7, True, 8]), ('control layout', [[3, 2, 2, 3, 1, 1, 1, 1, 1], 10], [0, True, 1])], [('regression: left clamp', [[1, 1, 1, 1], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 1, 1, 3], -1], [5, True, 6]), ('regression: left clamp', [[1, 1, 1, 1, 0, 3, 3], -1], [3, True, 4]), ('regression: left clamp', [[2, 2, 1, 0, 0, 0, 0], -2], [2, True, 3]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 1, 1, 1], 8], [1, False, 1]), ('control layout', [[2, 2, 2, 1, 1, 1, 1], 15], [2, True, 3])], [('regression: left clamp', [[3, 3, 3, 3, 1, 1, 1], -2], [6, True, 7]), ('regression: left clamp', [[2, 2, 2, 2, 2, 2], -1], [0, False, 0]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('partial-repair probe', [[1, 1, 1, 1], 1], [3, False, 3]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('control layout', [[2, 2, 2, 2, 2, 4, 4, 4], 8], [4, False, 4]), ('control layout', [[0, 0, 0, 3], 7], [3, False, 3])], [('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('click left of RTL line', [[1, 1, 1], -1], [2, True, 3]), ('regression: left clamp', [[3, 3, 1, 1, 1, 1], -1], [5, True, 6]), ('regression: left clamp', [[2, 2, 4, 4, 4, 4, 4, 2, 2], -1], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 2, 2, 2, 2, 0, 1, 1], 20], [8, False, 8]), ('control layout', [[2, 2, 2, 2], 7], [3, True, 4])], [('regression: left clamp', [[0, 0, 1, 1, 0], -1], [0, False, 0]), ('regression: left clamp', [[1, 1, 1, 1, 1, 3, 1, 1], -2], [7, True, 8]), ('regression: left clamp', [[0, 0, 1, 1, 1, 1, 1], -1], [0, False, 0]), ('regression: left clamp', [[0, 1], -2], [0, False, 0]), ('right half of RTL char', [[1, 1], 1], [1, False, 1]), ('click past end', [[0, 1, 1], 9], [1, False, 1]), ('control layout', [[3, 3, 3, 3, 3, 1, 2, 2, 2, 2], 11], [4, False, 4]), ('control layout', [[0, 0, 0, 1, 1, 1, 1, 3, 3], 3], [1, True, 2])]]
for label, args, expected in fixtures[N - 1]:
    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
click left of RTL line	`[2, True, 3]`	`[2, True, 3]`	Passed
regression: left clamp	`[1, True, 2]`	`[1, True, 2]`	Passed
partial-repair probe	`[7, True, 8]`	`[7, True, 8]`	Passed
regression: left clamp	`[6, False, 6]`	`[6, False, 6]`	Passed
right half of RTL char	`[1, False, 1]`	`[1, False, 1]`	Passed
click past end	`[1, False, 1]`	`[1, False, 1]`	Passed
control layout	`[7, True, 8]`	`[7, True, 8]`	Passed
control layout	`[0, True, 1]`	`[0, True, 1]`	Passed

SHA-256 / 4bbd26e98e3430b28e787f4609c5fa80d99c0a2aac94afa15ad061435b40368d

Verification & scope

A deterministic toy bidi model over stipulated class labels and integer levels; it is inspired by, but does not claim conformance to, any published algorithm. 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 2026-09-29T14:49:56.121697+00:00.

Case digest / f3714e565717afdd891cb90f6834b93a48f0b90a7a351e182dcca686ebe3ead7