diff --git a/README.md b/README.md
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+++ b/README.md
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+## factoradic
+
+Package factoradic provides support for [factorial numbers](https://xkcd.com/2835/).
diff --git a/factoradic.go b/factoradic.go
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+++ b/factoradic.go
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+// Package factoradic provides support for [factorial numbers](https://xkcd.com/2835/).
+package factoradic
+
+import "fmt"
+
+// A Number represents an integer value between 0 and 3628799₁₀ which renders in factorial base.
+type Number int32
+
+// String returns the factorial-base representation of a Number.
+func (n Number) String() string {
+	if n >= 0 {
+		nn := int32(n)
+		var result [9]byte
+		for base := int32(2); base <= 10; base++ {
+			result[10-base] = '0' + byte(nn%base)
+			nn /= base
+			if nn == 0 {
+				return string(result[10-base:])
+			}
+		}
+	}
+	return "###"
+}
+
+// ParseNumber interprets a string in factorial base and returns the corresponding value as a Number.
+// Returns a syntax error if any of the digits is outside of the allowable digits for that position,
+// or if the string length is longer than the maximum 9 digits allowed for factorial-base numbers.
+func ParseNumber(str string) (Number, error) {
+	if len(str) == 0 || len(str) > 9 {
+		return 0, syntaxError(str)
+	}
+	var (
+		result  int32
+		base    int32 = 2
+		cumBase int32 = 1
+	)
+	for i := len(str) - 1; i >= 0; i-- {
+		digit := int32(str[i] - '0')
+		if digit < 0 || digit >= base {
+			return 0, syntaxError(str)
+		}
+		result += digit * cumBase
+		cumBase *= base
+		base++
+	}
+	return Number(result), nil
+}
+
+func syntaxError(s string) error {
+	return fmt.Errorf("factoradic.Parse: parsing %q: invalid syntax", s)
+}
diff --git a/factoradic_test.go b/factoradic_test.go
new file mode 100644
index 0000000..2bca819
--- /dev/null
+++ b/factoradic_test.go
@@ -0,0 +1,116 @@
+package factoradic
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestNumber(t *testing.T) {
+
+	var tests = []struct {
+		base10        int32
+		factorialBase string
+	}{
+		{base10: -1, factorialBase: "###"},
+		{base10: 0, factorialBase: "0"},
+		{base10: 1, factorialBase: "1"},
+		{base10: 2, factorialBase: "10"},
+		{base10: 3, factorialBase: "11"},
+		{base10: 4, factorialBase: "20"},
+		{base10: 5, factorialBase: "21"},
+		{base10: 6, factorialBase: "100"},
+		{base10: 7, factorialBase: "101"},
+		{base10: 21, factorialBase: "311"},
+		{base10: 22, factorialBase: "320"},
+		{base10: 23, factorialBase: "321"},
+		{base10: 24, factorialBase: "1000"},
+		{base10: 25, factorialBase: "1001"},
+		{base10: 5038, factorialBase: "654320"},
+		{base10: 5039, factorialBase: "654321"},
+		{base10: 5040, factorialBase: "1000000"},
+		{base10: 999998, factorialBase: "266251210"},
+		{base10: 999999, factorialBase: "266251211"},
+		{base10: 1000000, factorialBase: "266251220"},
+		{base10: 1000001, factorialBase: "266251221"},
+		{base10: 3628799, factorialBase: "987654321"},
+		{base10: 3628800, factorialBase: "###"},
+	}
+
+	for _, test := range tests {
+		want := test.factorialBase
+		got := Number(test.base10).String()
+		if want != got {
+			t.Errorf("(%d) want %s, got %s", test.base10, want, got)
+		}
+	}
+}
+
+func TestParseNumber(t *testing.T) {
+
+	var tests = []struct {
+		base10        int32
+		factorialBase string
+		err           bool
+	}{
+		{factorialBase: "0", base10: 0},
+		{factorialBase: "1", base10: 1},
+		{factorialBase: "10", base10: 2},
+		{factorialBase: "11", base10: 3},
+		{factorialBase: "20", base10: 4},
+		{factorialBase: "21", base10: 5},
+		{factorialBase: "100", base10: 6},
+		{factorialBase: "101", base10: 7},
+		{factorialBase: "311", base10: 21},
+		{factorialBase: "320", base10: 22},
+		{factorialBase: "321", base10: 23},
+		{factorialBase: "1000", base10: 24},
+		{factorialBase: "1001", base10: 25},
+		{factorialBase: "654320", base10: 5038},
+		{factorialBase: "654321", base10: 5039},
+		{factorialBase: "1000000", base10: 5040},
+		{factorialBase: "266251210", base10: 999998},
+		{factorialBase: "266251211", base10: 999999},
+		{factorialBase: "266251220", base10: 1000000},
+		{factorialBase: "266251221", base10: 1000001},
+		{factorialBase: "987654321", base10: 3628799},
+		{factorialBase: "", err: true},
+		{factorialBase: "2", err: true},
+		{factorialBase: "102", err: true},
+		{factorialBase: "Z", err: true},
+		{factorialBase: "1111111111", err: true},
+		{factorialBase: "30", err: true},
+		{factorialBase: "400", err: true},
+	}
+
+	for _, test := range tests {
+		want := Number(test.base10)
+		got, err := ParseNumber(test.factorialBase)
+		if !test.err && err != nil {
+			t.Errorf("(%s) unexpected error %v", test.factorialBase, err)
+			continue
+		}
+		if test.err {
+			if err == nil {
+				t.Errorf("(%s) expected error, got base 10 value %d", test.factorialBase, got)
+			}
+			continue
+		}
+		if want != got {
+			t.Errorf("(%s) want base 10 value %d, got %d", test.factorialBase, want, got)
+		}
+	}
+}
+
+func ExampleNumber() {
+	var n Number = 23
+	fmt.Printf("Your number is %s.", n)
+	// Output:
+	// Your number is 321.
+}
+
+func ExampleParseNumber() {
+	n, _ := ParseNumber("321")
+	fmt.Printf("Your number in base 10 is %d.", n)
+	// Output:
+	// Your number in base 10 is 23.
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..378b8f9
--- /dev/null
+++ b/go.mod
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+module github.com/ders/factoradic
+
+go 1.21