Backport ferrilab#114 to 0.20.3

CHANGELOG.md

@@ -4,6 +4,7 @@ All notable changes will be documented in this file.
 
 This document is written according to the [Keep a Changelog][kac] style.
 
+1. [0.20.3](#0203)
 1. [0.20.2](#0202)
 1. [0.20.1](#0201)
 1. [0.20.0](#0200)
@@ -59,6 +60,10 @@ This document is written according to the [Keep a Changelog][kac] style.
 1. [0.2.0](#020)
 1. [0.1.0](#010)
 
+## 0.20.3
+
+Backports [#114].
+
 ## 0.20.2
 
 Fixed overly vague `funty` dependency, as the `1.2` release moved `BITS` from
@@ -1129,6 +1134,7 @@ Initial implementation and release.
 [@Fotosmile]: https://github.com/Fotosmile
 [@GeorgeGkas]: https://github.com/GeorgeGkas
 [@ImmemorConsultrixContrarie]: https://github.com/ImmemorConsultrixContrarie
+[@VilleHallivuori]: https://github.com/VilleHallivuori
 [@arucil]: https://github.com/arucil
 [@caelunshun]: https://github.com/caelunshun
 [@geq1t]: https://github.com/geq1t
@@ -1163,6 +1169,7 @@ Initial implementation and release.
 [Issue #69]: https://github.com/myrrlyn/bitvec/issues/69
 [Issue #75]: https://github.com/myrrlyn/bitvec/issues/75
 [Issue #83]: https://github.com/myrrlyn/bitvec/issues/83
+[Issue #114]: https://github.com/myrrlyn/bitvec/issues/114
 [Pull Request #34]: https://github.com/myrrlyn/bitvec/pull/34
 [Pull Request #41]: https://github.com/myrrlyn/bitvec/pull/41
 [Pull Request #68]: https://github.com/myrrlyn/bitvec/pull/68

Cargo.toml

@@ -6,7 +6,7 @@
 
 [package]
 name = "bitvec"
-version = "0.20.2"
+version = "0.20.3"
 authors = [
 	"myrrlyn <self@myrrlyn.dev>",
 ]

src/slice/specialization.rs

@@ -170,9 +170,9 @@ where T: BitStore
 	pub(crate) fn sp_iter_ones_last(&self) -> Option<usize> {
 		let mut out = match self.len() {
 			0 => return None,
-			n => n - 1,
+			n => n,
 		};
-		match self.domain() {
+		(|| match self.domain() {
 			Domain::Enclave { head, elem, tail } => {
 				let val = (Lsb0::mask(head, tail) & elem.load_value()).value();
 				let dead_bits = T::Mem::BITS - tail.value();
@@ -213,7 +213,8 @@ where T: BitStore
 
 				None
 			},
-		}
+		})()
+		.map(|idx| idx - 1)
 	}
 
 	/// Seeks the index of the first `0` bit in the bit-slice.
@@ -267,9 +268,9 @@ where T: BitStore
 	pub(crate) fn sp_iter_zeros_last(&self) -> Option<usize> {
 		let mut out = match self.len() {
 			0 => return None,
-			n => n - 1,
+			n => n,
 		};
-		match self.domain() {
+		(|| match self.domain() {
 			Domain::Enclave { head, elem, tail } => {
 				let val = (Lsb0::mask(head, tail) & !elem.load_value()).value();
 				let dead_bits = T::Mem::BITS - tail.value();
@@ -310,7 +311,8 @@ where T: BitStore
 
 				None
 			},
-		}
+		})()
+		.map(|idx| idx - 1)
 	}
 }
 

tests/issues.rs

@@ -1,6 +1,7 @@
 //! Test cases for defect reports.
 
-#[cfg(feature = "alloc")]
+#![allow(clippy::unusual_byte_groupings)]
+
 use bitvec::prelude::*;
 
 /** Test case for [Issue #10], opened by [@overminder].
@@ -271,3 +272,48 @@ fn issue_77() {
 	// are all `false`!!!
 	assert_eq!(&[&true; N], last_few.as_slice());
 }
+
+/** Test case for [Issue #114], opened by [@VilleHallivuori].
+
+This report describes an overflowing-subtraction error encountered when
+attempting to find the last index of an un/set bit in a bit-slice that does not
+have any.
+
+This is not a surprising crash: the reverse gallop by indices through a slice is
+a cumbersome operation that is fraught with potential for failure, thanks to the
+half-open nature of indices within a length.
+
+The fix turned out to be incredibly simple: defer the derement operation (to go
+from len to last valid index) from the *start* of the search to the *end*.
+
+I am not confident in the categorical correctness of this solution, but it fixes
+the provided test cases and does not break the existing tests.
+
+I will need to devise test cases that thoroughly check all possible branches of
+the galloping searches, but for now, this is an improvement over the existing
+behavior, so I am going to call it sufficient for the bug as reported, publish
+patches, and await further reports.
+
+[Issue #114]: https://github.com/bitvecto-rs/bitvec/issues/114
+[@VilleHallivuori]: https://github.com/VilleHallivuori
+**/
+#[test]
+fn issue_114() {
+	let one_zero = bits![0];
+	let one_one = bits![1];
+
+	assert_eq!(one_zero.count_zeros(), 1);
+	assert_eq!(one_zero.count_ones(), 0);
+	assert_eq!(one_one.count_zeros(), 0);
+	assert_eq!(one_one.count_ones(), 1);
+
+	assert!(one_zero.first_one().is_none());
+	assert!(one_zero.last_one().is_none());
+	assert!(one_one.first_zero().is_none());
+	assert!(one_one.last_zero().is_none());
+
+	assert_eq!(one_zero.first_zero(), Some(0));
+	assert_eq!(one_zero.last_zero(), Some(0));
+	assert_eq!(one_one.first_one(), Some(0));
+	assert_eq!(one_one.last_one(), Some(0));
+}