From ef0f14a51291fa6c0c1e8b8321ca7c07d446ba8e Mon Sep 17 00:00:00 2001
From: Kyle Isom <kyle@imap.cc>
Date: Mon, 2 Mar 2020 17:11:55 -0800
Subject: [PATCH] Add global config package.

This is a simple config system that I find myself wanting a lot.
---
 config/config.go         | 84 ++++++++++++++++++++++++++++++++++++++++
 config/config_test.go    | 66 +++++++++++++++++++++++++++++++
 config/testdata/test.env |  2 +
 3 files changed, 152 insertions(+)
 create mode 100644 config/config.go
 create mode 100644 config/config_test.go
 create mode 100644 config/testdata/test.env

diff --git a/config/config.go b/config/config.go
new file mode 100644
index 0000000..dd4596a
--- /dev/null
+++ b/config/config.go
@@ -0,0 +1,84 @@
+// Package config implements a simple global configuration system that
+// supports a file with key=value pairs and environment variables. Note
+// that the config system is global.
+package config
+
+import (
+	"bufio"
+	"log"
+	"os"
+	"strings"
+)
+
+// NB: Rather than define a singleton type, everything is defined at
+// the top-level
+
+var (
+	vars   = map[string]string{}
+	prefix = ""
+)
+
+// SetEnvPrefix sets the prefix for all environment variables; it's
+// assumed to not be needed for files.
+func SetEnvPrefix(pfx string) {
+	prefix = pfx
+}
+
+func addLine(line string) {
+	if strings.HasPrefix(line, "#") || line == "" {
+		return
+	}
+
+	lineParts := strings.SplitN(line, "=", 2)
+	if len(lineParts) != 2 {
+		log.Print("skipping line: ", line)
+		return // silently ignore empty keys
+	}
+
+	lineParts[0] = strings.TrimSpace(lineParts[0])
+	lineParts[1] = strings.TrimSpace(lineParts[1])
+	vars[lineParts[0]] = lineParts[1]
+}
+
+// LoadFile scans the file at path for key=value pairs and adds them
+// to the configuration.
+func LoadFile(path string) error {
+	file, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		addLine(line)
+	}
+
+	if err = scanner.Err(); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Get retrieves a value from either a configuration file or the
+// environment. Note that values from a file will override environment
+// variables.
+func Get(key string) string {
+	if v, ok := vars[key]; ok {
+		return v
+	}
+	return os.Getenv(prefix + key)
+}
+
+// GetDefault retrieves a value from either a configuration file or
+// the environment. Note that value from a file will override
+// environment variables. If a value isn't found (e.g. Get returns an
+// empty string), the default value will be used.
+func GetDefault(key, def string) string {
+	if v := Get(key); v != "" {
+		return v
+	}
+	return def
+}
diff --git a/config/config_test.go b/config/config_test.go
new file mode 100644
index 0000000..6d40c99
--- /dev/null
+++ b/config/config_test.go
@@ -0,0 +1,66 @@
+package config
+
+import (
+	"os"
+	"testing"
+)
+
+const (
+	testFilePath = "testdata/test.env"
+
+	// Keys
+	kOrder   = "ORDER"
+	kSpecies = "SPECIES"
+	kName    = "COMMON_NAME"
+
+	// Env
+	eOrder   = "corvus"
+	eSpecies = "corvus corax"
+	eName    = "northern raven"
+
+	// File
+	fOrder   = "stringiformes"
+	fSpecies = "strix aluco"
+	// Name isn't set in the file to test fall through.
+)
+
+func init() {
+	os.Setenv(kOrder, eOrder)
+	os.Setenv(kSpecies, eSpecies)
+	os.Setenv(kName, eName)
+}
+
+func TestLoadEnvOnly(t *testing.T) {
+	order := Get(kOrder)
+	species := Get(kSpecies)
+	if order != eOrder {
+		t.Errorf("want %s, have %s", eOrder, order)
+	}
+
+	if species != eSpecies {
+		t.Errorf("want %s, have %s", eSpecies, species)
+	}
+}
+
+func TestLoadFile(t *testing.T) {
+	err := LoadFile(testFilePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	order := Get(kOrder)
+	species := Get(kSpecies)
+	name := Get(kName)
+
+	if order != fOrder {
+		t.Errorf("want %s, have %s", fOrder, order)
+	}
+
+	if species != fSpecies {
+		t.Errorf("want %s, have %s", fSpecies, species)
+	}
+
+	if name != eName {
+		t.Errorf("want %s, have %s", eName, name)
+	}
+}
diff --git a/config/testdata/test.env b/config/testdata/test.env
new file mode 100644
index 0000000..5d66691
--- /dev/null
+++ b/config/testdata/test.env
@@ -0,0 +1,2 @@
+ORDER=stringiformes
+SPECIES=strix aluco