Added day 1.

.gitignore

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+.dub
+docs.json
+__dummy.html
+docs/
+/adventofcode2021
+adventofcode2021.so
+adventofcode2021.dylib
+adventofcode2021.dll
+adventofcode2021.a
+adventofcode2021.lib
+adventofcode2021-test-*
+*.exe
+*.o
+*.obj
+*.lst

dub.json

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+{
+	"authors": [
+		"Andrew Lalis"
+	],
+	"copyright": "Copyright © 2021, Andrew Lalis",
+	"description": "Advent of Code 2021 attempts.",
+	"license": "MIT",
+	"name": "adventofcode2021"
+}

source/app.d

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+import std.stdio;
+
+import day1.part1;
+import day1.part2;
+
+void main() {
+	slidingSum();
+}

source/day1/part1.d

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+module day1.part1;
+
+import std.stdio;
+import util.fileutils;
+
+void sonarSweep() {
+    int increaseCount = 0;
+    auto values = readInts("source/day1/input.txt");
+    int previousValue = values[0];
+    foreach(value; values[1..$]) {
+        if (value > previousValue) {
+            increaseCount++;
+        }
+        previousValue = value;
+    }
+    writefln("%d", increaseCount);
+}

source/day1/part2.d

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+module day1.part2;
+
+import std.stdio;
+import util.fileutils;
+
+void slidingSum() {
+    int[] values = readInts("source/day1/input.txt");
+    int previousSum = values[0] + values[1] + values[2];
+    int increaseCount = 0;
+    foreach (i; 1 .. (values.length - 2)) {
+        int currentSum = values[i] + values[i + 1] + values[i + 2];
+        bool increase = false;
+        if (currentSum > previousSum) {
+            increaseCount++;
+            increase = true;
+        }
+        previousSum = currentSum;
+    }
+    writefln("%d", increaseCount);
+}

source/util/fileutils.d

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+module util.fileutils;
+
+import std.file;
+import std.string;
+import std.algorithm;
+import std.array;
+import std.conv;
+
+int[] readInts(string filename) {
+    return readText("source/day1/input.txt")
+        .split("\n")
+        .map!(s => s.strip())
+        .filter!(s => s.length > 0)
+        .map!(s => s.to!int)
+        .array;
+}