feat: implement env file parser

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Go
+      uses: actions/setup-go@v5
+      with:
+        go-version: '1.22.x'
+    - name: Test
+      run: go test -v -race -cover -buildvcs ./...

LICENSE

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+MIT License
+
+Copyright (c) 2024 Sameer Jadav
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

Makefile

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+# Change these variables as necessary.
+PACKAGE_PATH := ./...
+
+# ==================================================================================== #
+# HELPERS
+# ==================================================================================== #
+
+## help: print this help message
+.PHONY: help
+help:
+	@echo 'Usage:'
+	@sed -n 's/^##//p' ${MAKEFILE_LIST} | column -t -s ':' |  sed -e 's/^/ /'
+
+# ==================================================================================== #
+# QUALITY CONTROL
+# ==================================================================================== #
+
+## test: run all tests with coverage
+.PHONY: test
+test:
+	@echo "Running tests..."
+	@go test -v -race -cover -buildvcs ${PACKAGE_PATH}
+
+# ==================================================================================== #
+# DEVELOPMENT
+# ==================================================================================== #
+
+## tidy: format code and tidy modfile
+.PHONY: tidy
+tidy:
+	@echo "Formatting code..."
+	@gofumpt -l -w .
+	@echo "Tidying Go mod..."
+	@go mod tidy -v

README.md

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+# GoEnvParse
+
+[![Go Reference](https://pkg.go.dev/badge/github.com/SameerJadav/go-envparse.svg)](https://pkg.go.dev/github.com/SameerJadav/go-envparse) [![CI](https://github.com/SameerJadav/go-envparse/actions/workflows/ci.yml/badge.svg)](https://github.com/SameerJadav/go-envparse/actions/workflows/ci.yml)
+
+GoEnvParse is a Go package for parsing environment variables from `.env` files. It provides a simple and efficient way to load environment variables from `.env` file into your Go applications.
+
+## Features
+
+- Parse environment files from any `io.Reader` source
+- Handling of quoted values (double quotes, single quotes, and backticks)
+- Variable expansion in non-quoted and double-quoted values
+- Error reporting with line numbers for invalid syntax
+
+## Parsing Details
+
+- Double-quoted values are unescaped, including unicode characters
+- Single-quoted and backtick-quoted values are treated as literal strings
+- Variable expansion is performed in non-quoted and double-quoted values
+- Does not support multiline values
+
+## Installation
+
+```shell
+go get github.com/SameerJadav/go-envparse
+```
+
+## Usage
+
+```go
+package main
+
+import (
+	"log"
+	"os"
+
+	"github.com/SameerJadav/go-envparse"
+)
+
+func main() {
+	file, err := os.Open(".env")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer file.Close()
+
+	env, err := envparse.Parse(file)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for key, value := range env {
+		os.Setenv(key, value)
+	}
+}
+```
+
+## Contributing
+
+Contributions are welcome. Please open an issue or submit a pull request.
+
+## License
+
+GoEnvParse is open-source and available under the [MIT License](./LICENSE).

go.mod

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+module github.com/SameerJadav/go-envparse
+
+go 1.22.5

parser.go

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+package envparse
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// Parse reads an env file from io.Reader, returning a map of keys-value pairs and an error.
+//
+// Only double-quoted values are escaped. Single-quoted and backquoted values
+// are treated as literal strings. Variable expansion (${...} and $...)
+// is performed in non-quoted and double-quoted values.
+//
+// Note: This function does not support multiline values.
+// Each key-value pair must be on a single line.
+func Parse(r io.Reader) (map[string]string, error) {
+	result := make(map[string]string)
+	scanner := bufio.NewScanner(r)
+	var lineNumber int
+	var err error
+
+	for scanner.Scan() {
+		lineNumber++
+
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" || line[0] == '#' || !strings.Contains(line, "=") {
+			continue
+		}
+
+		line = strings.TrimPrefix(line, "export ")
+
+		key, value, ok := strings.Cut(line, "=")
+		if !ok {
+			continue
+		}
+
+		key = strings.TrimSpace(key)
+		if key == "" {
+			continue
+		}
+
+		value = strings.TrimSpace(value)
+
+		if quote, start, end, ok := isQuoted(value); ok {
+			switch quote {
+			case '"':
+				value, err = strconv.Unquote(value[start : end+1])
+				if err != nil {
+					return nil, fmt.Errorf("failed to unquote value at line %d: %w", lineNumber, err)
+				}
+				value = expandVariables(value, result)
+			case '`':
+				value, err = strconv.Unquote(value[start : end+1])
+				if err != nil {
+					return nil, fmt.Errorf("failed to unquote value at line %d: %w", lineNumber, err)
+				}
+			case '\'':
+				value = value[start+1 : end]
+			}
+		} else {
+			if i := strings.IndexByte(value, '#'); i >= 0 {
+				value = strings.TrimSpace(value[:i])
+			}
+			value = expandVariables(value, result)
+		}
+
+		result[key] = value
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("error reading input: %w", err)
+	}
+
+	return result, nil
+}
+
+func isQuoted(value string) (byte, int, int, bool) {
+	if len(value) < 2 {
+		return 0, -1, -1, false
+	}
+
+	if quote := value[0]; quote == '"' || quote == '\'' || quote == '`' {
+		for i := len(value) - 1; i > 0; i-- {
+			if value[i] == quote && value[i-1] != '\\' {
+				return quote, 0, i, true
+			}
+		}
+		return quote, 0, -1, false
+	}
+
+	return 0, -1, -1, false
+}
+
+func expandVariables(value string, result map[string]string) string {
+	return os.Expand(value, func(key string) string {
+		if val, ok := result[key]; ok {
+			return val
+		}
+		return os.Getenv(key)
+	})
+}

parser_test.go

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+package envparse
+
+import (
+	"os"
+	"testing"
+)
+
+func TestParse(t *testing.T) {
+	expected := map[string]string{
+		"SIMPLE_VAR":                          "value",
+		"EMPTY":                               "",
+		"EMPTY_SINGLE_QUOTES":                 "",
+		"EMPTY_DOUBLE_QUOTES":                 "",
+		"EMPTY_BACKTICKS":                     "",
+		"QUOTED_VAR":                          "value",
+		"SINGLE_QUOTED_VAR":                   "value",
+		"BACKQUOTED_VAR":                      "value",
+		"DOUBLE_QUOTES_SPACED":                "    double quotes    ",
+		"SINGLE_QUOTES_SPACED":                "    single quotes    ",
+		"BACKQUOTE_SPACED":                    "    back quotes    ",
+		"UNQUOTED_WITH_SPACES":                "this has spaces",
+		"NESTED_IN_DOUBLE":                    "This is a 'single quote' and a `backtick` inside double quotes",
+		"NESTED_IN_SINGLE":                    "This is a \"double quote\" and a `backtick` inside single quotes",
+		"NESTED_IN_BACKTICK":                  "This is a \"double quote\" and a 'single quote' inside backticks",
+		"ESCAPED_IN_DOUBLE":                   "This has \"escaped double quotes\"",
+		"ESCAPED_IN_SINGLE":                   "This has \\'escaped single quotes\\'",
+		"MIXED_QUOTES":                        "Double with 'single' and \"escaped double\" and `backtick`",
+		"UNQUOTED_NEWLINE":                    "This is a line\\nAnd this is another line",
+		"DOUBLE_QUOTED_NEWLINE":               "This is in double quotes\nThis is a new line in double quotes",
+		"SINGLE_QUOTED_NEWLINE":               "This is in single quotes\\nThis is a new line in single quotes",
+		"BACKTICK_QUOTED_NEWLINE":             "This is in backticks\\nThis is a new line in backticks",
+		"VARIABLE_EXPANSION":                  "value/expansion",
+		"NESTED_EXPANSION":                    "value/expansion/nested",
+		"VARIABLE_EXPANSION_ALT":              "value/expansion",
+		"NESTED_EXPANSION_ALT":                "value/expansion/nested",
+		"DOUBLEQUOTED_VARIABLE_EXPANSION":     "value/expansion",
+		"DOUBLEQUOTED_NESTED_EXPANSION":       "value/expansion/nested",
+		"DOUBLEQUOTED_VARIABLE_EXPANSION_ALT": "value/expansion",
+		"DOUBLEQUOTED_NESTED_EXPANSION_ALT":   "value/expansion/nested",
+		"SINGLEQUOTED_VARIABLE_EXPANSION":     "${SIMPLE_VAR}/expansion",
+		"SINGLEQUOTED_NESTED_EXPANSION":       "${VARIABLE_EXPANSION}/nested",
+		"SINGLEQUOTED_VARIABLE_EXPANSION_ALT": "$SIMPLE_VAR/expansion",
+		"SINGLEQUOTED_NESTED_EXPANSION_ALT":   "$VARIABLE_EXPANSION/nested",
+		"BACKQUOTED_VARIABLE_EXPANSION":       "${SIMPLE_VAR}/expansion",
+		"BACKQUOTED_NESTED_EXPANSION":         "${VARIABLE_EXPANSION}/nested",
+		"BACKQUOTED_VARIABLE_EXPANSION_ALT":   "$SIMPLE_VAR/expansion",
+		"BACKQUOTED_NESTED_EXPANSION_ALT":     "$VARIABLE_EXPANSION/nested",
+		"UNMATCHED_DOUBLEQUOTE":               "\"value",
+		"UNMATCHED_SINGLEQUOTE":               "'value",
+		"UNMATCHED_BACKQUOTE":                 "`value",
+		"INLINE_COMMENTS":                     "value",
+		"INLINE_COMMENTS_DOUBLE_QUOTES":       "inline comments outside of #doublequotes",
+		"INLINE_COMMENTS_SINGLE_QUOTES":       "inline comments outside of #singlequotes",
+		"INLINE_COMMENTS_BACKQUOTES":          "inline comments outside of #backticks",
+		"EXPORTED_VAR":                        "value",
+		"EQUAL_SIGNS":                         "equals==",
+		"SPACED_KEY":                          "value",
+	}
+
+	file, err := os.Open("test.env")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file.Close()
+
+	result, err := Parse(file)
+	if err != nil {
+		t.Fatalf("Parse returned unexpected error: %v", err)
+	}
+
+	if len(result) != len(expected) {
+		t.Errorf("Map size mismatch: got %d entries, want %d entries", len(result), len(expected))
+	}
+
+	for expectedKey, expectedValue := range expected {
+		actualValue, ok := result[expectedKey]
+		if !ok {
+			t.Errorf("Missing key: %s is not present in the result map, but it was expected", expectedKey)
+		}
+		if actualValue != expectedValue {
+			t.Errorf("Value mismatch for key %s: got %s, want %s", expectedKey, actualValue, expectedValue)
+		}
+	}
+
+	for key, value := range result {
+		if _, ok := expected[key]; !ok {
+			t.Errorf("Unexpected key-value pair: %s=%s is present in the result map, but it was not expected.", key, value)
+		}
+	}
+}