package mcfg

import (
	"context"
	"encoding/json"
)

// ParamValue describes a value for a parameter which has been parsed by a
// Source.
type ParamValue struct {
	Name  string
	Path  []string
	Value json.RawMessage
}

// Source parses ParamValues out of a particular configuration source, given a
// sorted set of possible Params to parse, and the Context from with the Params
// were extracted.
//
// It's possible for Parsing to affect the Context itself, for example in the
// case of sub-commands. For this reason Parse can return a Context, which will
// get used for subsequent Parse commands inside, and then returned from,
// Populate.
//
// Source should not return ParamValues which were not explicitly set to a value
// by the configuration source.
//
// The returned []ParamValue may contain duplicates of the same Param's value.
// in which case the later value takes precedence. It may also contain
// ParamValues which do not correspond to any of the passed in Params. These
// will be ignored in Populate.
type Source interface {
	Parse(context.Context, []Param) (context.Context, []ParamValue, error)
}

// ParamValues is simply a slice of ParamValue elements, which implements Parse
// by always returning itself as-is.
type ParamValues []ParamValue

// Parse implements the method for the Source interface.
func (pvs ParamValues) Parse(ctx context.Context, _ []Param) (context.Context, []ParamValue, error) {
	return ctx, pvs, nil
}

// Sources combines together multiple Source instances into one. It will call
// Parse on each element individually. Values from later Sources take precedence
// over previous ones.
type Sources []Source

// Parse implements the method for the Source interface.
func (ss Sources) Parse(ctx context.Context, params []Param) (context.Context, []ParamValue, error) {
	var pvs []ParamValue
	for _, s := range ss {
		var innerPVs []ParamValue
		var err error
		if ctx, innerPVs, err = s.Parse(ctx, params); err != nil {
			return nil, nil, err
		}
		pvs = append(pvs, innerPVs...)
	}
	return ctx, pvs, nil
}