FAILURE MAP
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FA-62281 / Currency rounding and FX conversion / Open access

The dispensed amount is rounded to the nearest note · case 01

The ATM dispenses more foreign currency than the customer requested.

Verified by executionVariant 1 · 6 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Note units are rounded half-even instead of floored.

THE FAILURE

Note units are rounded half-even instead of floored.

Unsuccessful approach: Rounding half-down still dispenses an extra note when the fraction exceeds one half.

Case contract

solve(home_minor, rate, note, fee_minor): a request of home_minor home cents at rate foreign major units per home major unit is dispensed as the largest multiple of the smallest note not exceeding the converted amount (floor). If nothing can be dispensed return 'ERR:below-min-note'. The customer is charged the dispensed foreign amount converted back at the same rate, rounded UP to a home cent, plus fee_minor. Return [foreign_dispensed, charged_minor].

Why this case matters

Currency amounts must be rounded at the right stage and in the right unit, or ledgers, quotes and settlements drift by minor units.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
from decimal import Decimal, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_FLOOR, ROUND_CEILING, ROUND_DOWN, ROUND_UP, ROUND_HALF_DOWN
N = 1
observations = []
def solve(home_minor, rate, note, fee_minor):
    r = Decimal(rate)
    home = Decimal(home_minor) / 100
    units = (home * r / note).to_integral_value(rounding=ROUND_HALF_EVEN)
    foreign = int(units) * note
    if foreign == 0: return 'ERR:below-min-note'
    charged = int((Decimal(foreign) / r * 100).to_integral_value(rounding=ROUND_CEILING)) + fee_minor
    return [foreign, charged]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression note-rounding 1', (1000, '0.92', 5, 0), [5, 544]),
  ('regression note-rounding 2', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801])],
 [('regression note-rounding 1', (130189, '821.097', 5, 0), [1068975, 130189]),
  ('regression note-rounding 2', (97345, '30.034', 10, 0), [29230, 97324]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319])],
 [('regression note-rounding 1', (190855, '53583.55', 5, 500), [102266880, 191355]),
  ('regression note-rounding 2', (42899, '157086.46', 10000, 500), [67380000, 43394]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: euro fives', (1000, '0.92', 5, 0), [5, 544]),
  ('control: below note', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170])],
 [('regression note-rounding 1', (86470, '264.5122', 10000, 500), [220000, 83672]),
  ('regression note-rounding 2', (2489, '60494.94', 10, 0), [1505710, 2489]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319])],
 [('regression note-rounding 1', (16458, '4444.030', 20, 0), [731380, 16458]),
  ('regression note-rounding 2', (100634, '12628.590', 10000, 250), [12700000, 100816]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: euro fives', (1000, '0.92', 5, 0), [5, 544]),
  ('control: below note', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170])]]
for label, args, expected in cases[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 fixtureActualExpectedOutcome
regression note-rounding 1[10, 1087][5, 544]Failed
regression note-rounding 2[5, 794]ERR:below-min-noteFailed
control: yen notes[30000, 20319][30000, 20319]Passed
control: exact multiple[100, 11170][100, 11170]Passed
control: big notes[1350000, 99816][1350000, 99816]Passed
control: sterling tens[60, 7801][60, 7801]Passed

SHA-256 / d27d67ca7b90497034b659bea75702531e4532d0eaecb970b7e2522ea6ce1ac7

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
from decimal import Decimal, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_FLOOR, ROUND_CEILING, ROUND_DOWN, ROUND_UP, ROUND_HALF_DOWN
N = 1
observations = []
def solve(home_minor, rate, note, fee_minor):
    r = Decimal(rate)
    home = Decimal(home_minor) / 100
    units = (home * r / note).to_integral_value(rounding=ROUND_HALF_DOWN)
    foreign = int(units) * note
    if foreign == 0: return 'ERR:below-min-note'
    charged = int((Decimal(foreign) / r * 100).to_integral_value(rounding=ROUND_CEILING)) + fee_minor
    return [foreign, charged]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression note-rounding 1', (1000, '0.92', 5, 0), [5, 544]),
  ('regression note-rounding 2', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801])],
 [('regression note-rounding 1', (130189, '821.097', 5, 0), [1068975, 130189]),
  ('regression note-rounding 2', (97345, '30.034', 10, 0), [29230, 97324]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319])],
 [('regression note-rounding 1', (190855, '53583.55', 5, 500), [102266880, 191355]),
  ('regression note-rounding 2', (42899, '157086.46', 10000, 500), [67380000, 43394]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: euro fives', (1000, '0.92', 5, 0), [5, 544]),
  ('control: below note', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170])],
 [('regression note-rounding 1', (86470, '264.5122', 10000, 500), [220000, 83672]),
  ('regression note-rounding 2', (2489, '60494.94', 10, 0), [1505710, 2489]),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170]),
  ('control: big notes', (100000, '1352.5', 10000, 0), [1350000, 99816]),
  ('control: sterling tens', (7777, '0.7893', 10, 199), [60, 7801]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319])],
 [('regression note-rounding 1', (16458, '4444.030', 20, 0), [731380, 16458]),
  ('regression note-rounding 2', (100634, '12628.590', 10000, 250), [12700000, 100816]),
  ('control: yen notes', (20000, '151.37', 1000, 500), [30000, 20319]),
  ('control: euro fives', (1000, '0.92', 5, 0), [5, 544]),
  ('control: below note', (500, '0.92', 5, 250), 'ERR:below-min-note'),
  ('control: exact multiple', (10870, '0.92', 10, 300), [100, 11170])]]
for label, args, expected in cases[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 fixtureActualExpectedOutcome
regression note-rounding 1[10, 1087][5, 544]Failed
regression note-rounding 2[5, 794]ERR:below-min-noteFailed
control: yen notes[30000, 20319][30000, 20319]Passed
control: exact multiple[100, 11170][100, 11170]Passed
control: big notes[1350000, 99816][1350000, 99816]Passed
control: sterling tens[60, 7801][60, 7801]Passed

SHA-256 / 16b2a1fcacf03ca512538fe976ed854013c4a6a7bdebe1d35bef5ea5119df1b4

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This mechanism has 6 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

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Verification & scope

A deterministic, bounded teaching model with a stipulated toy contract; it makes no claim of conformance to any real regulation, standard, or institution's rules. 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 2026-09-29T14:47:03.148611+00:00.

Case digest / 6ca53899b23ca6019b5676ff3ad44ba62da471c7449ace28b78483f2b9217dbc