diff --git a/day21/day.go b/day21/day.go
new file mode 100644
index 0000000..d748add
--- /dev/null
+++ b/day21/day.go
@@ -0,0 +1,183 @@
+package day21
+
+import (
+	"math"
+	"strconv"
+	"strings"
+	"tools"
+)
+
+var armor = map[string]map[string]int{
+	"none": {
+		"cost":  0,
+		"armor": 0,
+	},
+	"Leather": {
+		"cost":  13,
+		"armor": 1,
+	},
+	"Chainmail": {
+		"cost":  31,
+		"armor": 2,
+	},
+	"Splintmail": {
+		"cost":  53,
+		"armor": 3,
+	},
+	"Bandedmail": {
+		"cost":  75,
+		"armor": 4,
+	},
+	"Platemail": {
+		"cost":  102,
+		"armor": 5,
+	},
+}
+
+var weapons = map[string]map[string]int{
+	"Dagger": {
+		"cost":   8,
+		"damage": 4,
+	},
+	"Shortsword": {
+		"cost":   10,
+		"damage": 5,
+	},
+	"Warhammer": {
+		"cost":   25,
+		"damage": 6,
+	},
+	"Longsword": {
+		"cost":   40,
+		"damage": 7,
+	},
+	"Greataxe": {
+		"cost":   74,
+		"damage": 8,
+	},
+}
+
+var rings = map[string]map[string]int{
+	"none1": {
+		"cost":   0,
+		"damage": 0,
+		"armor":  0,
+	},
+	"none2": {
+		"cost":   0,
+		"damage": 0,
+		"armor":  0,
+	},
+	"Damage +1": {
+		"cost":   25,
+		"damage": 1,
+		"armor":  0,
+	},
+	"Damage +2": {
+		"cost":   50,
+		"damage": 2,
+		"armor":  0,
+	},
+	"Damage +3": {
+		"cost":   100,
+		"damage": 3,
+		"armor":  0,
+	},
+	"Armor +1": {
+		"cost":   20,
+		"damage": 0,
+		"armor":  1,
+	},
+	"Armor +2": {
+		"cost":   40,
+		"damage": 0,
+		"armor":  2,
+	},
+	"Armor +3": {
+		"cost":   80,
+		"damage": 0,
+		"armor":  3,
+	},
+}
+
+func fight(playerHp, playerPower, playerArmor, bossHp, bossPower, bossArmor int) bool {
+	playerStrike := float64(tools.Max(1, playerPower-bossArmor))
+	bossStrike := float64(tools.Max(1, bossPower-playerArmor))
+
+	return math.Ceil(float64(bossHp)/playerStrike) <= math.Ceil(float64(playerHp)/bossStrike)
+}
+
+func Part1(puzzle tools.AoCPuzzle) interface{} {
+	input := puzzle.GetInputArray()
+	hpLine := strings.Split(input[0], ": ")
+	powerLine := strings.Split(input[1], ": ")
+	armorLine := strings.Split(input[2], ": ")
+	bossHp, _ := strconv.Atoi(hpLine[1])
+	bossPower, _ := strconv.Atoi(powerLine[1])
+	bossArmor, _ := strconv.Atoi(armorLine[1])
+
+	playerHp := 100
+	playerPower := 0
+	playerArmor := 0
+
+	goldSpent := 0
+
+	minGold := math.MaxInt32
+	// completely ignoring rings for p1 as they are way too expensive in any case
+	for _, weaponStats := range weapons {
+		for _, armorStats := range armor {
+			goldSpent = weaponStats["cost"] + armorStats["cost"]
+			playerPower = weaponStats["damage"]
+			playerArmor = armorStats["armor"]
+
+			if fight(playerHp, playerPower, playerArmor, bossHp, bossPower, bossArmor) && goldSpent < minGold {
+				minGold = goldSpent
+			}
+		}
+	}
+
+	// REVISIT: The "correct" answer according to AoC website was 78, but that doesn't work!
+	// The only way to spent 78 Gold is by buying a Warhammer and the Splintmail
+	// That put the player damage at 6(weapon)-1(bossArmor) == 5
+	// And the boss damage becomes 8(input)-3(armor) == 5
+	// But the boss has 104 HP and the player only 100.
+	// Leaving the player dead after 20 Rounds with the boss still having 4 HP
+	return minGold
+}
+
+func Part2(puzzle tools.AoCPuzzle) interface{} {
+	input := puzzle.GetInputArray()
+	hpLine := strings.Split(input[0], ": ")
+	powerLine := strings.Split(input[1], ": ")
+	armorLine := strings.Split(input[2], ": ")
+	bossHp, _ := strconv.Atoi(hpLine[1])
+	bossPower, _ := strconv.Atoi(powerLine[1])
+	bossArmor, _ := strconv.Atoi(armorLine[1])
+
+	playerHp := 100
+	playerPower := 0
+	playerArmor := 0
+
+	goldSpent := 0
+
+	maxGold := 0
+	for _, weaponStats := range weapons {
+		for _, armorStats := range armor {
+			for ring1Name, ring1Stats := range rings {
+				for ring2Name, ring2Stats := range rings {
+					if ring1Name == ring2Name {
+						continue
+					}
+					goldSpent = weaponStats["cost"] + armorStats["cost"] + ring1Stats["cost"] + ring2Stats["cost"]
+					playerPower = weaponStats["damage"] + ring1Stats["damage"] + ring2Stats["damage"]
+					playerArmor = armorStats["armor"] + ring1Stats["armor"] + ring2Stats["armor"]
+
+					if !fight(playerHp, playerPower, playerArmor, bossHp, bossPower, bossArmor) && goldSpent > maxGold {
+						maxGold = goldSpent
+					}
+				}
+			}
+		}
+	}
+	return maxGold
+}
diff --git a/inputs/21 b/inputs/21
new file mode 100644
index 0000000..e0dc07d
--- /dev/null
+++ b/inputs/21
@@ -0,0 +1,3 @@
+Hit Points: 104
+Damage: 8
+Armor: 1
\ No newline at end of file
diff --git a/inputs/21_test b/inputs/21_test
new file mode 100644
index 0000000..e0dc07d
--- /dev/null
+++ b/inputs/21_test
@@ -0,0 +1,3 @@
+Hit Points: 104
+Damage: 8
+Armor: 1
\ No newline at end of file
diff --git a/main.go b/main.go
index 2d47c5b..08e5ae4 100644
--- a/main.go
+++ b/main.go
@@ -21,6 +21,7 @@ import (
 	"aoc2015/day18"
 	"aoc2015/day19"
 	"aoc2015/day20"
+	"aoc2015/day21"
 	"flag"
 	"fmt"
 	"os"
@@ -58,6 +59,7 @@ func initDayFunctions() {
 		18: {1: day18.Part1, 2: day18.Part2},
 		19: {1: day19.Part1, 2: day19.Part2},
 		20: {1: day20.Part1, 2: day20.Part2},
+		21: {1: day21.Part1, 2: day21.Part2},
 	}
 }