First solution

inputs/01/test.txt

@@ -0,0 +1,6 @@
+3  4
+4  3
+2  5
+1  3
+3  9
+3  3

solutions/01.zig

@@ -0,0 +1,91 @@
+const std = @import("std");
+const print = std.debug.print;
+
+pub fn main() !void {
+    var arr1: [1000]u32 = undefined;
+    var arr2: [1000]u32 = undefined;
+
+    // Reading the input...
+    // Making use of example here: https://cookbook.ziglang.cc/01-01-read-file-line-by-line.html
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const file = try std.fs.cwd().openFile("../inputs/01/real.txt", .{});
+    defer file.close();
+
+    // Wrap the file reader in a buffered reader.
+    // Since it's usually faster to read a bunch of bytes at once.
+    var buf_reader = std.io.bufferedReader(file.reader());
+    const reader = buf_reader.reader();
+
+    var line = std.ArrayList(u8).init(allocator);
+    defer line.deinit();
+
+    const writer = line.writer();
+    var line_no: usize = 0;
+
+    while (reader.streamUntilDelimiter(writer, '\n', null)) {
+        // Clear the line so we can reuse it.
+        defer line.clearRetainingCapacity();
+        line_no += 1;
+
+        print("{d}--{s}\n", .{ line_no, line.items });
+
+        const values = parseLineToNumbers(line.items);
+        arr1[line_no - 1] = values.first;
+        arr2[line_no - 1] = values.second;
+    } else |err| switch (err) {
+        error.EndOfStream => { // end of file
+            if (line.items.len > 0) {
+                line_no += 1;
+                print("{d}--{s}\n", .{ line_no, line.items });
+            }
+        },
+        else => return err, // Propagate error
+    }
+
+    // Everything below this is the actual logic
+    const stdout = std.io.getStdOut().writer();
+    std.mem.sort(u32, &arr1, {}, comptime std.sort.asc(u32));
+    std.mem.sort(u32, &arr2, {}, comptime std.sort.asc(u32));
+
+    var sum: u32 = 0;
+    for (0..arr1.len) |index| {
+        const val1 = arr1[index];
+        const val2 = arr2[index];
+        if (val1 > val2) {
+            sum += (val1 -% val2);
+        } else {
+            sum += (val2 -% val1);
+        }
+    }
+    try stdout.print("{}", .{sum});
+}
+
+fn parseLineToNumbers(line: []u8) struct { first: u32, second: u32 } {
+    var first: u32 = 0;
+    var second: u32 = 0;
+    var isInFirst = true;
+    for (line) |char| {
+        if (char == ' ') {
+            print("found a space\n", .{});
+            isInFirst = false;
+            continue;
+        }
+        if (isInFirst) {
+            print("first\n", .{});
+            first += (char - 48);
+            print("{}\n", .{first});
+            first *= 10;
+            print("{}\n", .{first});
+        } else {
+            print("second\n", .{});
+            second += (char - 48);
+            print("{}\n", .{second});
+            second *= 10;
+            print("{}\n", .{second});
+        }
+    }
+    return .{ .first = first / 10, .second = second / 10 };
+}