scaled-ui: Floor all conversions to 0 #159
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#### Problem As pointed out at anza-xyz/agave#4663 (comment), the current scaling behavior is problematic for money amounts, because it uses the default Rust formatting of "round to nearest, ties to even" when producing a UI amount. This behavior can "overvalue" what's in someone's account. #### Summary of changes Floor all conversions to 0 (AKA truncate).
steveluscher
approved these changes
Feb 3, 2025
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I get that this is just taking the ScaledUiConfig and manipulating it faithfully, but I thought that negative multipliers were prohibited by program policy. I didn't want to have to think if rounding negative numbers toward zero was the right thing to do (ie. don't overstate how much you owe) or if we should be rounding toward negative infinity (ie. don't understate how much you owe).
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| // negative truncation | ||
| let config = ScaledUiAmountConfig { | ||
| multiplier: PodF64::from(-0.99), | ||
| new_multiplier_effective_timestamp: UnixTimestamp::from(1), | ||
| ..Default::default() | ||
| }; | ||
| // This is really -0.99999... but it gets truncated | ||
| let ui_amount = config.amount_to_ui_amount(101, 2, 0).unwrap(); | ||
| assert_eq!(ui_amount, "-0.99"); |
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Wait, I thought negative multipliers were illegal.
token-2022/program/src/extension/scaled_ui_amount/processor.rs
Lines 32 to 36 in 14c06ee
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Oh yeah, you're right! I'll remove those tests
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Problem
As pointed out at
anza-xyz/agave#4663 (comment), the current scaling behavior is problematic for money amounts, because it uses the default Rust formatting of "round to nearest, ties to even" when producing a UI amount. This behavior can "overvalue" what's in someone's account.
Summary of changes
Floor all conversions to 0 (AKA truncate).