commit d161ab35a77983d7599d9cdb0d43cefa3b1fc372
Author: Lucas F. <lucas@lucasf.dev>
Date:   Thu Jul 16 11:08:24 2026 -0300

    initial

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e4f39af
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,41 @@
+## Go
+
+# If you prefer the allow list template instead of the deny list, see community template:
+# https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
+#
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+bin
+
+# Test binary, built with `go test -c`
+*.test
+
+# Code coverage profiles and other test artifacts
+*.out
+coverage.*
+*.coverprofile
+profile.cov
+
+# Dependency directories (remove the comment below to include it)
+# vendor/
+
+# development notes
+/devnotes
+
+# Go workspace file
+go.work
+go.work.sum
+
+# env file
+.env
+
+# compiled binary at root
+/main
+
+# Editor/IDE
+# .idea/
+# .vscode/
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..e8f66c6
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,10 @@
+.PHONY: build test clean
+
+build:
+	@mkdir -p bin && go build -o bin/cpfgen .
+
+test:
+	@go test -v ./...
+
+clean:
+	@rm -rf bin
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..c52b04b
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module git.lucasf.xyz/public/cpfgen
+
+go 1.26.4
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..e8410af
--- /dev/null
+++ b/main.go
@@ -0,0 +1,57 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math/rand/v2"
+	"strings"
+)
+
+func main() {
+	onlyNum := flag.Bool("num", false, "generate only numbers (no formatting)")
+	flag.Parse()
+
+	cpf := generateCPF()
+
+	if *onlyNum {
+		fmt.Println(cpf)
+	} else {
+		fmt.Printf("%s.%s.%s-%s\n", cpf[:3], cpf[3:6], cpf[6:9], cpf[9:])
+	}
+}
+
+func generateCPF() string {
+	digits := make([]int, 11)
+
+	for i := range 9 {
+		digits[i] = rand.IntN(10)
+	}
+
+	sum := 0
+	for i := range 9 {
+		sum += digits[i] * (10 - i)
+	}
+	rem := sum % 11
+	if rem < 2 {
+		digits[9] = 0
+	} else {
+		digits[9] = 11 - rem
+	}
+
+	sum = 0
+	for i := range 10 {
+		sum += digits[i] * (11 - i)
+	}
+	rem = sum % 11
+	if rem < 2 {
+		digits[10] = 0
+	} else {
+		digits[10] = 11 - rem
+	}
+
+	var result strings.Builder
+	for _, d := range digits {
+		fmt.Fprintf(&result, "%d", d)
+	}
+	return result.String()
+}
diff --git a/main_test.go b/main_test.go
new file mode 100644
index 0000000..2513a13
--- /dev/null
+++ b/main_test.go
@@ -0,0 +1,87 @@
+package main
+
+import (
+	"strconv"
+	"testing"
+)
+
+func TestGenerateCPFLength(t *testing.T) {
+	cpf := generateCPF()
+	if len(cpf) != 11 {
+		t.Fatalf("expected length 11, got %d: %s", len(cpf), cpf)
+	}
+}
+
+func TestGenerateCPFCheckDigits(t *testing.T) {
+	for range 100 {
+		cpf := generateCPF()
+		if !isValidCPF(cpf) {
+			t.Fatalf("invalid CPF generated: %s", cpf)
+		}
+	}
+}
+
+func TestGenerateCPFAllDigits(t *testing.T) {
+	seen := make(map[string]bool)
+	for range 1000 {
+		cpf := generateCPF()
+		if seen[cpf] {
+			t.Fatal("duplicate CPF generated")
+		}
+		seen[cpf] = true
+	}
+}
+
+func isValidCPF(cpf string) bool {
+	if len(cpf) != 11 {
+		return false
+	}
+
+	digits := make([]int, 11)
+	for i, ch := range cpf {
+		d, err := strconv.Atoi(string(ch))
+		if err != nil {
+			return false
+		}
+		digits[i] = d
+	}
+
+	allEqual := true
+	for i := 1; i < 11; i++ {
+		if digits[i] != digits[0] {
+			allEqual = false
+			break
+		}
+	}
+	if allEqual {
+		return false
+	}
+
+	sum := 0
+	for i := range 9 {
+		sum += digits[i] * (10 - i)
+	}
+	rem := sum % 11
+	var d1 int
+	if rem < 2 {
+		d1 = 0
+	} else {
+		d1 = 11 - rem
+	}
+	if d1 != digits[9] {
+		return false
+	}
+
+	sum = 0
+	for i := range 10 {
+		sum += digits[i] * (11 - i)
+	}
+	rem = sum % 11
+	var d2 int
+	if rem < 2 {
+		d2 = 0
+	} else {
+		d2 = 11 - rem
+	}
+	return d2 == digits[10]
+}