Add shrinking a la delta debugging #345
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,161 @@ | ||
| // Copyright 2025 PRAGMA | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
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| use std::fmt::Debug; | ||
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| // Andreas Zeller's delta debugging (`ddmin`) algorithm from the paper | ||
| // "Simplifying and Isolating Failure-Inducing Input" (2002). | ||
| // | ||
| // Basically tries to bisect the input (git's bisect algorithm uses the same technique). Will first | ||
| // try throwing away half of the input, but if that fails it will throw away smaller and smaller | ||
| // parts until it finds the smallest counter example. | ||
| pub fn shrink<A: Debug + Clone, B: Debug>( | ||
| test: impl Fn(&[A]) -> B, | ||
| mut input: Vec<A>, | ||
| error_predicate: impl Fn(&B) -> bool, | ||
| ) -> (Vec<A>, B, u32) { | ||
| let mut number_of_shrinks = 0; | ||
| let mut last_error: B; | ||
| let result = test(&input); | ||
| if error_predicate(&result) { | ||
| last_error = result; | ||
| } else { | ||
| panic!( | ||
| "shrink, error predicate doesn't hold for initial input: '{:?}'", | ||
| input | ||
| ) | ||
| } | ||
| let mut n = 2; | ||
| while input.len() >= 2 { | ||
| let mut start = 0; | ||
| let subset_length = input.len() / n; | ||
| let mut some_complement_is_failing = false; | ||
| while start < input.len() { | ||
|
Contributor
There was a problem hiding this comment. when
Contributor
Author
There was a problem hiding this comment. I think removing a single input element from the end is correct... |
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| let mut complement: Vec<A> = Vec::new(); | ||
| complement.extend_from_slice(&input[..start]); | ||
| if start + subset_length < input.len() { | ||
| complement.extend_from_slice(&input[start + subset_length..]); | ||
| } | ||
| // NOTE: that if we get a different error than the expected one, we treat it as a | ||
| // passing test. | ||
| let result = test(&complement); | ||
| if error_predicate(&result) { | ||
| number_of_shrinks += 1; | ||
| last_error = result; | ||
| input = complement; | ||
| n = n.max(2) - 1; | ||
|
stevana marked this conversation as resolved.
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| some_complement_is_failing = true; | ||
| break; | ||
| } | ||
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| start += subset_length; | ||
| } | ||
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| if !some_complement_is_failing { | ||
| if n == input.len() { | ||
| break; | ||
| } | ||
| n = (n * 2).min(input.len()) | ||
| } | ||
| } | ||
| (input, last_error, number_of_shrinks) | ||
| } | ||
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| #[cfg(test)] | ||
| mod test { | ||
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| use super::*; | ||
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| #[test] | ||
| fn test_shrink_failing() { | ||
| let failing_input = vec![1, 2, 3, 42, 5, 6]; | ||
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| let test = |input: &[u8]| { | ||
| // println!("input: {:?}", input); | ||
| // input: [1, 2, 3, 42, 5, 6] | ||
| // input: [42, 5, 6] | ||
| // input: [5, 6] | ||
| // input: [42, 6] | ||
| // input: [6] | ||
| // input: [42] | ||
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| if input.contains(&42) { | ||
| Err("Found 42".to_string()) | ||
| } else { | ||
| Ok(()) | ||
| } | ||
| }; | ||
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| assert_eq!( | ||
| shrink(test, failing_input, |err| *err | ||
| == Err("Found 42".to_string())), | ||
| (vec![42], Err("Found 42".to_string()), 3) | ||
| ); | ||
| } | ||
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| #[test] | ||
| fn test_shrink_unresolved() { | ||
| let failing_input = vec![1, 2, 3, 42, 5, 6]; | ||
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| let test = |input: &[u8]| { | ||
| // println!("input: {:?}", input); | ||
| // input: [1, 2, 3, 42, 5, 6] | ||
| // input: [42, 5, 6] <-- NOTE: This will return a different error message than the one | ||
| // we expect, which ddmin treats as a passing test. | ||
| // input: [1, 2, 3] | ||
| // input: [2, 3, 42, 5, 6] | ||
| // input: [3, 42, 5, 6] | ||
| // input: [5, 6] | ||
| // input: [3, 42] | ||
| // input: [42] | ||
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| if input.len() == 3 && input.contains(&5) { | ||
| assert_eq!(input, vec![42, 5, 6]); | ||
| return Err("Found 5".to_string()); | ||
| }; | ||
| if input.contains(&42) { | ||
| Err("Found 42".to_string()) | ||
| } else { | ||
| Ok(()) | ||
| } | ||
| }; | ||
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| assert_eq!( | ||
| shrink(test, failing_input, |err| *err | ||
| == Err("Found 42".to_string())), | ||
| (vec![42], Err("Found 42".to_string()), 4) | ||
| ) | ||
| } | ||
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| #[test] | ||
| #[should_panic( | ||
| expected = "shrink, error predicate doesn't hold for initial input: '[1, 2, 3]'" | ||
| )] | ||
| fn test_shrink_passing() { | ||
| let failing_input = vec![1, 2, 3]; | ||
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| let test = |input: &[u8]| { | ||
| if input.contains(&4) { | ||
| Err("Found 4".to_string()) | ||
| } else { | ||
| Ok(()) | ||
| } | ||
| }; | ||
| assert_eq!( | ||
| shrink(test, failing_input, |err| *err | ||
| == Err("Found 4".to_string())), | ||
| (vec![4], Err("Found 4".to_string()), 0) | ||
| ) | ||
| } | ||
| } | ||
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