Binary Search First Occurrence

Find the first copy of a duplicated target by recording matches and continuing to search the left half.

Algorithm

Basic Implementation

basic.rs

fn main() {
	let arr = [1, 2, 4, 4, 4, 7, 9];
	let target = 4;
	let mut lo: i32 = 0;
	let mut hi: i32 = (arr.len() as i32) - 1;
	let mut result: i32 = -1;
	while lo <= hi {
		let mid = lo + (hi - lo) / 2;
		if arr[mid as usize] == target {
			result = mid;
			hi = mid - 1;
		} else if arr[mid as usize] < target {
			lo = mid + 1;
		} else {
			hi = mid - 1;
		}
	}
	println!("{}", result);
}

Complexity

Time: O(log n)
Space: O(1)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-first`.

cross-language comparison This Rust DSA version keeps the same data and final output as every other DSA book in this wave.