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fn main() {
// PARAMETERS
let context_lines = 2;
let needle = "oo";
let haystack = "Every face, every shop,
bedroom window, public-house, and
dark square is a picture
feverishly turned--in search of what?
It is the same with books.
What do we seek
through millions of pages?";
// INITIALIZATION
let mut tags: Vec<usize> = Vec::new(); // <1> `tags` holds line numbers where matches occur
let mut ctx: Vec<Vec<(usize, String)>> = Vec::new(); // <2> `ctx` contains a vector per match to hold that match's context lines
// PASS 1
for (i, line) in haystack.lines().enumerate() { // <3> iterate through the lines, recording line numbers where matches are encountered
if line.contains(needle) {
tags.push(i);
let v = Vec::with_capacity(2*context_lines + 1); // <4> <5> `Vec::with_capacity(_n_)` reserves space for _n_ items
ctx.push(v);
}
}
if tags.len() == 0 { // <6> When there are no matches, exit early
return;
}
// PASS 2
for (i, line) in haystack.lines().enumerate() { // <7> For each tag, at every line, check to see if we are nearby a match. When we are, add that line to the relevant `Vec<T>` within `ctx`.
for (j, tag) in tags.iter().enumerate() {
let lower_bound = tag.saturating_sub(context_lines); // <8> `usize.saturating_sub()` returns 0, rather than underflowing
let upper_bound = tag + context_lines;
if (i >= lower_bound) && (i <= upper_bound) {
let line_as_string = String::from(line); // <9> Copy `line` into a new `String` and store that locally for each match
let local_ctx = (i, line_as_string);
ctx[j].push(local_ctx);
}
}
}
// OUTPUT
for local_ctx in ctx.iter() {
for &(i, ref line) in local_ctx.iter() { // <10> `ref line` informs the compiler that we wish to borrow this value, rather than move it. These two terms are explained fully later in later chapters.
let line_num = i + 1;
println!("{}: {}", line_num, line);
}
}
}
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