Format initial string to specific precision, so rounding works. #4159
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The logic in our double-based sprintf logic attempts to round values manually by inspecting a long-form version of the double. If we need to round, and the next digit is a five, it will round toward zero (truncate) iff that five is the last digit in the long unrounded string. This appears to have been an attempt to mimic C printf's behavior of rounding "true half" to even in the presence of inaccurately-represented IEEE754 decimals. This commit changes the pre-formatting to actually format with the specified precision, allowing NumberFormat's default HALF_EVEN logic to to the work for us. Fixes jruby#4157.
This was referenced Oct 30, 2016
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The logic in our double-based sprintf logic attempts to round
values manually by inspecting a long-form version of the double.
If we need to round, and the next digit is a five, it will round
toward zero (truncate) iff that five is the last digit in the long
unrounded string. This appears to have been an attempt to mimic
C printf's behavior of rounding "true half" to even in the
presence of inaccurately-represented IEEE754 decimals.
This commit changes the pre-formatting to actually format with the
specified precision, allowing NumberFormat's default HALF_EVEN
logic to to the work for us.
Fixes #4157.