Could round(n) and BigDecimal(bd, digits) return self when nothing changes? (1 → 0 objects, ~2.5x on round)
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调研方向
Read the implementations of BigDecimal#round, #floor, #ceil, and #truncate, plus BigDecimal(bd, digits), and the test_BigDecimal_bug7522 test mentioned in the issue. First compare their current behavior with the proposed identity-preserving cases, including the stated rounding modes and edge cases. Done means deciding whether either change is acceptable and, if so, adding coverage for value equivalence and object identity.
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描述
Hi, a question before sending a PR, since it touches object identity.
In a money app most round(2) calls receive a value that already has at most two decimals: line totals, prices read from a decimal(10,2) column, and Rails' own apply_scale on every decimal attribute it casts. Each of those allocates a new, equal BigDecimal. On 2,000 real orders from our database, round(n) calls that drop no digit account for about 50k of the 326k BigDecimals allocated. BigDecimal(bd, digits) is similar: it always copies, even though it never rounds a BigDecimal argument today (the /* TODO: rounding */ is still there), and Rails calls it on every decimal attribute read.
I prototyped both returning self when nothing would change:
round(n),floor(n),ceil(n)andtruncate(n)with n >= 1, when no digit is dropped:BigDecimal("129.90").round(2)goes from 168 to 63 ns and 1 to 0 objects. n < 1 keeps returning an Integer as it does now.BigDecimal(bd, digits)whenbdhas no more significant digits thandigits: 52 to 29 ns, 1 to 0 objects. A longerbdis still copied, so the TODO stays open.
Both gave identical results to master on 5M fuzzed cases (every rounding mode and half: option, n from -3 to 40, ±0, NaN, ±Infinity, huge exponents), comparing to_s, sign, precision_scale and _dump. Ruby 4.0.9, macOS arm64.
The catch is that x.round(2).equal?(x) becomes true, as Integer#round already behaves, and test_BigDecimal_bug7522 asserts assert_not_same(bd, BigDecimal(bd, 1)), so that test would have to change for the second one. BigDecimals are frozen, so nothing can be mutated through the shared object. For the Rails case I'm also preparing a fix on the Rails side, which works with every bigdecimal version, but our own round(2) calls would only benefit from a change here.
Would you accept returning self in these cases, for one of them or both, or would you rather keep a fresh object every time?
🤖 Generated with Claude Code, but prompted, read, and verified by @czj, a caring human :)
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