CFDate: return the ISO 8601 week of the year - #128
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CFAbsoluteTimeGetWeekOfYear returned d / 7 % 52, which ignored the day of the week and could never reach week 53. Compute the ISO 8601 week number, where weeks start on Monday and week 1 contains the year first Thursday, so days near a year boundary fall in a week of the adjacent year.
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CFAbsoluteTimeGetWeekOfYear computed the week as d / 7 % 52. That ignores which day of the week the year starts on and can never return week 53, so almost every date came back with the wrong week (for example 2004-12-31 gave 0 instead of 53).
Compute the ISO 8601 week number instead: weeks start on Monday and week 1 is the week containing the year's first Thursday, so days at the start or end of a year can belong to a week of the adjacent year. A helper returns whether a year has 52 or 53 ISO weeks.
Tests/CFDate/weekofyear.m covers the first weeks of the year, the mid-January boundaries, and the year-end rollovers for both a 52-week and a 53-week year, verified against Apple CoreFoundation.