Added some comments to make le code clear.

source/day1/part1.d

@@ -7,6 +7,7 @@ void sonarSweep() {
     int increaseCount = 0;
     auto values = readInts("source/day1/input.txt");
     int previousValue = values[0];
+    // Iterate from value index 1 to the end of the list.
     foreach(value; values[1..$]) {
         if (value > previousValue) {
             increaseCount++;

source/day1/part2.d

@@ -7,6 +7,7 @@ void slidingSum() {
     int[] values = readInts("source/day1/input.txt");
     int previousSum = values[0] + values[1] + values[2];
     int increaseCount = 0;
+    // Iterate over indices 1 to 2 minus the length of the array.
     foreach (i; 1 .. (values.length - 2)) {
         int currentSum = values[i] + values[i + 1] + values[i + 2];
         if (currentSum > previousSum) increaseCount++;

source/day2/part1and2.d

@@ -5,12 +5,17 @@ import std.stdio;
 import std.string;
 import std.conv;
 import std.typecons;
+import std.traits;
 
-Tuple!(string, int) parseOp(char[] op) {
+/** 
+ * Parses a submarine instruction.
+ * Params:
+ *   op = The operation to parse.
+ * Returns: A tuple containing the operation string and the value.
+ */
+Tuple!(string, int) parseOp(S)(S op) @safe pure if (isSomeString!S) {
     auto parts = op.split();
-    int x = parts[1].to!int;
+    return tuple(parts[0].to!string, parts[1].to!int);
-    string d = parts[0].to!string;
-    return tuple(d, x);
 }
 
 void dive() {

source/util/fileutils.d

@@ -6,6 +6,12 @@ import std.algorithm;
 import std.array;
 import std.conv;
 
+/** 
+ * Reads a list of integers from a file, assuming one integer per line.
+ * Params:
+ *   filename = The name of the file to read, relative to the working dir.
+ * Returns: A list of integers.
+ */
 int[] readInts(string filename) {
     return readText("source/day1/input.txt")
         .split("\n")