diff --git a/day9/part1/.gitignore b/day9/part1/.gitignore
new file mode 100644
index 0000000..6f15a26
--- /dev/null
+++ b/day9/part1/.gitignore
@@ -0,0 +1,15 @@
+.dub
+docs.json
+__dummy.html
+docs/
+/day9-part1
+day9-part1.so
+day9-part1.dylib
+day9-part1.dll
+day9-part1.a
+day9-part1.lib
+day9-part1-test-*
+*.exe
+*.o
+*.obj
+*.lst
diff --git a/day9/part1/dub.json b/day9/part1/dub.json
new file mode 100644
index 0000000..b91781b
--- /dev/null
+++ b/day9/part1/dub.json
@@ -0,0 +1,12 @@
+{
+	"authors": [
+		"Johannes Loher"
+	],
+	"copyright": "Copyright © 2020, Johannes Loher",
+	"dependencies": {
+		"mir-algorithm": "~>3.10.12"
+	},
+	"description": "A minimal D application.",
+	"license": "MIT",
+	"name": "day9-part1"
+}
\ No newline at end of file
diff --git a/day9/part1/dub.selections.json b/day9/part1/dub.selections.json
new file mode 100644
index 0000000..1cd5164
--- /dev/null
+++ b/day9/part1/dub.selections.json
@@ -0,0 +1,7 @@
+{
+	"fileVersion": 1,
+	"versions": {
+		"mir-algorithm": "3.10.12",
+		"mir-core": "1.1.53"
+	}
+}
diff --git a/day9/part1/input b/day9/part1/input
new file mode 100644
index 0000000..fd6cff0
--- /dev/null
+++ b/day9/part1/input
@@ -0,0 +1,1000 @@
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diff --git a/day9/part1/source/app.d b/day9/part1/source/app.d
new file mode 100644
index 0000000..16ab47f
--- /dev/null
+++ b/day9/part1/source/app.d
@@ -0,0 +1,28 @@
+import std;
+
+import mir.combinatorics;
+
+void main()
+{
+    File("input", "r").byLine.map!(to!long)
+        .array.findFirstNumberNotSumOf2PreviousNumbers(25).writeln;
+}
+
+auto findFirstNumberNotSumOf2PreviousNumbers(long[] numbers, size_t windowSize)
+in(numbers.length > windowSize)
+{
+    return numbers.slide(windowSize + 1)
+        .map!(window => tuple(window[0 .. $ - 1].combinations(2).map!sum, window[windowSize]))
+        .find!(pair => !pair[0].canFind(pair[1]))
+        .front[1];
+}
+
+unittest
+{
+    long[] input = [
+        35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299,
+        277, 309, 576
+    ];
+
+    assert(input.findFirstNumberNotSumOf2PreviousNumbers(5) == 127);
+}
diff --git a/day9/part2/.gitignore b/day9/part2/.gitignore
new file mode 100644
index 0000000..21e97db
--- /dev/null
+++ b/day9/part2/.gitignore
@@ -0,0 +1,15 @@
+.dub
+docs.json
+__dummy.html
+docs/
+/day9-part2
+day9-part2.so
+day9-part2.dylib
+day9-part2.dll
+day9-part2.a
+day9-part2.lib
+day9-part2-test-*
+*.exe
+*.o
+*.obj
+*.lst
diff --git a/day9/part2/dub.json b/day9/part2/dub.json
new file mode 100644
index 0000000..d0f9849
--- /dev/null
+++ b/day9/part2/dub.json
@@ -0,0 +1,11 @@
+{
+	"authors": [
+		"Johannes Loher"
+	],
+	"copyright": "Copyright © 2020, Johannes Loher",
+	"dependencies": {
+		"mir-algorithm": "~>3.10.12"
+	},
+	"license": "MIT",
+	"name": "day9-part2"
+}
\ No newline at end of file
diff --git a/day9/part2/dub.selections.json b/day9/part2/dub.selections.json
new file mode 100644
index 0000000..1cd5164
--- /dev/null
+++ b/day9/part2/dub.selections.json
@@ -0,0 +1,7 @@
+{
+	"fileVersion": 1,
+	"versions": {
+		"mir-algorithm": "3.10.12",
+		"mir-core": "1.1.53"
+	}
+}
diff --git a/day9/part2/input b/day9/part2/input
new file mode 100644
index 0000000..fd6cff0
--- /dev/null
+++ b/day9/part2/input
@@ -0,0 +1,1000 @@
+21
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diff --git a/day9/part2/source/app.d b/day9/part2/source/app.d
new file mode 100644
index 0000000..699773a
--- /dev/null
+++ b/day9/part2/source/app.d
@@ -0,0 +1,62 @@
+import std;
+
+import mir.combinatorics;
+
+void main()
+{
+    File("input", "r").byLine.map!(to!long).array.breakXMASEncryption(25).writeln;
+}
+
+auto findFirstNumberNotSumOf2PreviousNumbers(long[] numbers, size_t windowSize)
+in(numbers.length > windowSize)
+{
+    return numbers.slide(windowSize + 1)
+        .map!(window => tuple(window[0 .. $ - 1].combinations(2).map!sum, window[windowSize]))
+        .find!(pair => !pair[0].canFind(pair[1]))
+        .front[1];
+}
+
+unittest
+{
+    long[] input = [
+        35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299,
+        277, 309, 576
+    ];
+
+    assert(input.findFirstNumberNotSumOf2PreviousNumbers(5) == 127);
+}
+
+auto findContiguousSubrangeWhichSumsTo(long[] input, long number)
+{
+    return iota(2, input.length + 1).map!(i => input.slide(i))
+        .joiner
+        .find!(it => it.sum == number)
+        .front;
+}
+
+unittest
+{
+    long[] input = [
+        35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299,
+        277, 309, 576
+    ];
+
+    assert(input.findContiguousSubrangeWhichSumsTo(127) == [15, 25, 47, 40]);
+}
+
+auto breakXMASEncryption(long[] input, size_t windowSize)
+{
+    auto invalidNumber = input.findFirstNumberNotSumOf2PreviousNumbers(windowSize);
+    auto contiguousSubrange = input.findContiguousSubrangeWhichSumsTo(invalidNumber);
+    return contiguousSubrange.minElement + contiguousSubrange.maxElement;
+}
+
+unittest
+{
+    long[] input = [
+        35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299,
+        277, 309, 576
+    ];
+
+    assert(input.breakXMASEncryption(5) == 62);
+}