Day 7 part 2 linear

Author: gaborhermann

day07/part1_linear.zig

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+const std = @import("std");
+
+/// This is a refactor of part1.zig that makes it easier to
+/// do part2 linearly.
+pub fn main() !void {
+    var file = try std.fs.cwd().openFile("input", .{});
+    defer file.close();
+
+    var buf_reader = std.io.bufferedReader(file.reader());
+    var in_stream = buf_reader.reader();
+
+    var read_buf: [1024 * 64]u8 = undefined;
+    const line = try in_stream.readUntilDelimiterOrEof(&read_buf, '\n');
+    var split = std.mem.split(line.?, ",");
+
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var positions = std.ArrayList(u32).init(allocator);
+    defer positions.deinit();
+
+    // Read all positions.
+    while (split.next()) |pos_str| {
+        const pos = try std.fmt.parseInt(u32, pos_str, 10);
+        try positions.append(pos);
+    }
+
+    var crabs: []u32 = positions.items;
+
+    // Sort.
+    std.sort.sort(u32, crabs, {}, comptime std.sort.asc(u32));
+
+    const len: u32 = @truncate(u32, crabs.len);
+    const min_pos: u32 = crabs[0];
+    const max_pos: u32 = crabs[len - 1];
+
+    var sum_dist_left: i64 = 0;
+    var sum_dist_right: i64 = 0;
+
+    for (crabs) |pos| {
+        sum_dist_right += pos - min_pos;
+    }
+
+    var min_fuel_pos: u32 = min_pos;
+    var min_fuel: i64 = sum_dist_left + sum_dist_right;
+
+    var pos: u32 = min_pos + 1;
+    var i: u32 = 0;
+    while (pos <= max_pos) : (pos += 1) {
+        while (pos > crabs[i]) {
+            i += 1;
+        }
+
+        const crabs_left: u32 = i;
+        const crabs_right: u32 = len - i;
+
+        // We are getting further from i crabs,
+        sum_dist_left += crabs_left;
+        // closer to (len - i) crabs.
+        sum_dist_right -= crabs_right;
+
+        const total_fuel = sum_dist_left + sum_dist_right;
+        if (min_fuel > total_fuel) {
+            min_fuel = total_fuel;
+            min_fuel_pos = pos;
+        }
+    }
+
+    const stdout = std.io.getStdOut().writer();
+    try stdout.print("{d}\n", .{min_fuel});
+}

day07/part2_linear.zig

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+const std = @import("std");
+
+/// This is a refactor of part2 that is linear.
+pub fn main() !void {
+    var file = try std.fs.cwd().openFile("input", .{});
+    defer file.close();
+
+    var buf_reader = std.io.bufferedReader(file.reader());
+    var in_stream = buf_reader.reader();
+
+    var read_buf: [1024 * 64]u8 = undefined;
+    const line = try in_stream.readUntilDelimiterOrEof(&read_buf, '\n');
+    var split = std.mem.split(line.?, ",");
+
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var positions = std.ArrayList(u32).init(allocator);
+    defer positions.deinit();
+
+    // Read all positions.
+    while (split.next()) |pos_str| {
+        const pos = try std.fmt.parseInt(u32, pos_str, 10);
+        try positions.append(pos);
+    }
+
+    var crabs: []u32 = positions.items;
+
+    // Sort.
+    std.sort.sort(u32, crabs, {}, comptime std.sort.asc(u32));
+
+    const len: u32 = @truncate(u32, crabs.len);
+    const min_pos: u32 = crabs[0];
+    const max_pos: u32 = crabs[len - 1];
+
+    var sum_dist_left: i64 = 0;
+    var sum_dist_right: i64 = 0;
+
+    var sum_fuel_left: i64 = 0;
+    var sum_fuel_right: i64 = 0;
+
+    for (crabs) |pos| {
+        const dist = pos - min_pos;
+        sum_dist_right += dist;
+        sum_fuel_right += (dist * (dist + 1)) / 2;
+    }
+
+    var min_fuel_pos: u32 = min_pos;
+    var min_fuel: i64 = sum_fuel_left + sum_fuel_right;
+
+    var pos: u32 = min_pos + 1;
+    var i: u32 = 0;
+    while (pos <= max_pos) : (pos += 1) {
+        while (pos > crabs[i]) {
+            i += 1;
+        }
+
+        const crabs_left: u32 = i;
+        const crabs_right: u32 = len - i;
+
+        // We are getting further from i crabs,
+        sum_dist_left += crabs_left;
+
+        // Crabs on the left would consume as much more
+        // fuel as their distance.
+        sum_fuel_left += sum_dist_left;
+
+        // Crabs on the right would consume as much less
+        // fuel as their previous distance.
+        sum_fuel_right -= sum_dist_right;
+
+        // We're getting closer to (len - i) crabs.
+        sum_dist_right -= crabs_right;
+
+        const total_fuel = sum_fuel_left + sum_fuel_right;
+        if (min_fuel > total_fuel) {
+            min_fuel = total_fuel;
+            min_fuel_pos = pos;
+        }
+    }
+
+    const stdout = std.io.getStdOut().writer();
+    try stdout.print("{d}\n", .{min_fuel});
+}