package mcfg

import (
	"fmt"
	"os"
	"strings"
)

// SourceEnv is a Source which will parse configuration from the process
// environment.
//
// Possible Env options are generated by joining a Param's Path and Name with
// underscores and making all characters uppercase, as well as changing all
// dashes to underscores.
//
//	ctx := mctx.New()
//	ctx = mctx.ChildOf(ctx, "foo")
//	ctx = mctx.ChildOf(ctx, "bar")
//	addr := mcfg.String(ctx, "srv-addr", "", "Some address")
//	// the Env option to fill addr will be "FOO_BAR_SRV_ADDR"
//
type SourceEnv struct {
	// In the format key=value. Defaults to os.Environ() if nil.
	Env []string

	// If set then all expected Env options must be prefixed with this string,
	// which will be uppercased and have dashes replaced with underscores like
	// all the other parts of the option names.
	Prefix string
}

func (env SourceEnv) expectedName(path []string, name string) string {
	out := strings.Join(append(path, name), "_")
	if env.Prefix != "" {
		out = env.Prefix + "_" + out
	}
	out = strings.Replace(out, "-", "_", -1)
	out = strings.ToUpper(out)
	return out
}

// Parse implements the method for the Source interface
func (env SourceEnv) Parse(params []Param) ([]ParamValue, error) {
	kvs := env.Env
	if kvs == nil {
		kvs = os.Environ()
	}

	pM := map[string]Param{}
	for _, p := range params {
		name := env.expectedName(p.Path, p.Name)
		pM[name] = p
	}

	pvs := make([]ParamValue, 0, len(kvs))
	for _, kv := range kvs {
		split := strings.SplitN(kv, "=", 2)
		if len(split) != 2 {
			return nil, fmt.Errorf("malformed environment kv %q", kv)
		}
		k, v := split[0], split[1]
		if p, ok := pM[k]; ok {
			pvs = append(pvs, ParamValue{
				Param: p,
				Value: p.fuzzyParse(v),
			})
		}
	}

	return pvs, nil
}