Fix too restrictive condition in Java RyuFloat impl.#244
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ulfjack merged 1 commit intoulfjack:masterfrom Jan 16, 2026
Merged
Fix too restrictive condition in Java RyuFloat impl.#244ulfjack merged 1 commit intoulfjack:masterfrom
ulfjack merged 1 commit intoulfjack:masterfrom
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When `e2 < 0`, current `RyuFloat` calculates `dvIsTrailingZeros` by the code below, which should be equivalent to `mv%2^(q-1) == 0`. ```java dvIsTrailingZeros = (q < FLOAT_MANTISSA_BITS) && (mv & ((1 << (q - 1)) - 1)) == 0; ``` https://github.com/ulfjack/ryu/blob/1264a946ba66eab320e927bfd2362e0c8580c42f/src/main/java/info/adams/ryu/RyuFloat.java#L207 If I understand correctly, `(q < FLOAT_MANTISSA_BITS)` is an optimization that works as follows: - The maximum value of `v` is `2^FLOAT_MANTISSA_BITS - 1` - Therefore, when `q` is greater than or equal to `FLOAT_MANTISSA_BITS`, `v` is never divisible by `2^q` - So, if `(q < FLOAT_MANTISSA_BITS)` is false, we can skip the `(mv & ((1 << (q - 1)) - 1)) == 0` check **However, I think it should be `(q - 1 < FLOAT_MANTISSA_BITS + 2)` because**: - We're checking if `mv` is divisible by `2^(q-1)`, not `2^q`. So the left-hand side should be `q - 1` - The maximum value of `mv` is larger than `2^FLOAT_MANTISSA_BITS - 1`: - `mv` is `4 * m`, where `m` is the 23-bit mantissa. This means `mv <= 4 * (2^23 - 1) = 2^25 - 4` - To check if `mv` is divisible by `2^(q-1)`, the condition `q < 23` is too restrictive, because `mv` can be divisible by up to `2^24` - Therefore, the right-hand side should be `FLOAT_MANTISSA_BITS + 2` --- Actually, a test from `f2s_test.cc`'s `lotsOfTrailingZeros` fails in the Java implementation: https://github.com/ulfjack/ryu/blob/1264a946ba66eab320e927bfd2362e0c8580c42f/ryu/tests/f2s_test.cc#L69 ``` 1) lotsOfTrailingZeros(info.adams.ryu.RyuFloatTest) org.junit.ComparisonFailure: expected:<0.002441406[2]> but was:<0.002441406[3]> ``` This is because: - `mv = 41943040`, `e2 = -34`, and `q = 23` - Therefore, `dvIsTrailingZeros = false` because `q < 23` is false (but it should be true) - By step 4, `dv` is reduced from `244140625` to `24414062` (with `lastRemovedDigit = 5`) - The condition `(dvIsTrailingZeros && (lastRemovedDigit == 5) && (dv % 2 == 0))` evaluates to false (but it should be true!), because `dvIsTrailingZeros = false` https://github.com/ulfjack/ryu/blob/1264a946ba66eab320e927bfd2362e0c8580c42f/src/main/java/info/adams/ryu/RyuFloat.java#L267-L269 As a result, it is rounded up to `24414063`, but it should be rounded to `24414062`.
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Thanks for the PR. I need some more time to review (I know, I know). Also need to fix the presubmit checks again... |
ulfjack
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Jan 6, 2026
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I'm still not sure about the -3, but let me approve this to see if we can get CI to run. (Sorry for the noisy emails; this might take a few more attempts.)
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I agree with the reasoning. Technically, it's safe to allow even larger values of q as long as we don't hit the 32-bit integer boundary. The C implementation does this: |
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fix #243