grep: keep only the requested number of before-context lines

src/context_buffer.rs

@@ -42,13 +42,16 @@ impl BufferedLine {
 /// provide a more optimized `ContextBuffer` when `mmap()` is available.
 pub struct ContextBuffer {
     slots: Vec<BufferedLine>,
+    /// Number of lines to retain. `slots` is rounded up to a power of two so
+    /// `push` can mask instead of divide, so it may be larger than this.
+    capacity: usize,
     head: usize,
     len: usize,
 }
 
 impl ContextBuffer {
     pub fn new(capacity: usize) -> Self {
-        let len = if capacity == 0 {
+        let slots = if capacity == 0 {
             0
         } else {
             capacity.next_power_of_two()
@@ -60,8 +63,9 @@ impl ContextBuffer {
                     line_number: 0,
                     byte_offset: 0,
                 };
-                len
+                slots
             ],
+            capacity,
             head: 0,
             len: 0,
         }
@@ -90,7 +94,7 @@ impl ContextBuffer {
         slot.byte_offset = byte_offset;
 
         self.head = self.head.wrapping_add(1);
-        self.len = (self.len + 1).min(self.slots.len());
+        self.len = (self.len + 1).min(self.capacity);
     }
 
     pub fn drain_iter(&mut self) -> impl Iterator<Item = &BufferedLine> {

tests/test_grep.rs

@@ -1050,6 +1050,26 @@ fn after_before_combined_context() {
         .stdout_only("b\nMATCH\nc\n");
 }
 
+#[test]
+fn before_context_is_not_rounded_to_power_of_two() {
+    // The context ring buffer rounds its slot count up to a power of two, but
+    // must still retain only the requested number of lines.
+    let input = "01\n02\n03\n04\n05\n06\n07\n08\n09\n10\nMM\n";
+
+    for (n, expected) in [
+        ("3", "08\n09\n10\nMM\n"),
+        ("5", "06\n07\n08\n09\n10\nMM\n"),
+        ("6", "05\n06\n07\n08\n09\n10\nMM\n"),
+        ("7", "04\n05\n06\n07\n08\n09\n10\nMM\n"),
+    ] {
+        let (_s, mut c) = ucmd();
+        c.args(&["-e", "MM", "-B", n])
+            .pipe_in(input)
+            .succeeds()
+            .stdout_only(expected);
+    }
+}
+
 #[test]
 fn num_shorthand_is_context() {
     // `-2` is shorthand for `-C 2`.