Find a pattern in a file and mark the position

find.zig

const std = @import("std");

const COLOR_RED = "\x1b[0;31m";
const COLOR_RESET = "\x1b[0m";

fn print_usage(exec: [*:0]u8) void {
    std.debug.print("Usage: {s} FILE PATTERN\n", .{exec});
}

fn string_contains_substring(allocator: std.mem.Allocator, string: []u8, substring: []u8) !std.ArrayList([2]usize) {
    var list = std.ArrayList([2]usize).init(allocator);

    var start: ?usize = null;
    var substring_index: usize = 0;

    for (string, 0..) |c, i| {
        if (c == substring[substring_index]) {
            substring_index += 1;

            if (substring_index == substring.len) {
                try list.append(.{ start orelse i, i });
                start = null;
                substring_index = 0;
                continue;
            }

            if (start == null) {
                start = i;
            }
        } else if (start != null) {
            start = null;
            substring_index = 0;
        }
    }

    return list;
}

pub fn main() !void {
    if (std.os.argv.len != 3) {
        print_usage(std.os.argv[0]);
        std.process.exit(1);
        unreachable;
    }

    const stdout = std.io.getStdOut().writer();

    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    defer {
        const status = gpa.deinit();
        if (status == .leak) @panic("there are leaks");
    }

    const args = try std.process.argsAlloc(allocator);
    defer std.process.argsFree(allocator, args);

    const file = try std.fs.cwd().openFile(args[1], .{});
    defer file.close();

    const reader = file.reader();
    while (true) {
        const line = reader.readUntilDelimiterAlloc(allocator, '\n', 1024 * 1024) catch break;
        defer allocator.free(line);
        const contains = try string_contains_substring(allocator, line, args[2]);
        defer contains.deinit();

        if (contains.items.len == 0) {
            try stdout.print("  {s}\n", .{line});
        } else {
            try stdout.print("> ", .{});
            var previous: usize = 0;
            for (contains.items) |element| {
                try stdout.print("{s}{s}{s}{s}", .{ line[previous..element[0]], COLOR_RED, line[element[0] .. element[1] + 1], COLOR_RESET });
                previous = element[1] + 1;
            }
            try stdout.print("{s}\n", .{line[previous..]});
        }
    }
}