package dagjson

import (
	"fmt"

	cid "github.com/ipfs/go-cid"
	"github.com/polydawn/refmt/shared"
	"github.com/polydawn/refmt/tok"

	ipld "github.com/ipld/go-ipld-prime"
	cidlink "github.com/ipld/go-ipld-prime/linking/cid"
)

// This drifts pretty far from the general unmarshal in the parent package:
//   - we know JSON never has length hints, so we ignore that field in tokens;
//   - we know JSON never has tags, so we ignore that field as well;
//   - we have dag-json's special sauce for detecting schemafree links
//      (and this unfortunately turns out to *significantly* convolute the first
//       several steps of handling maps, because it necessitates peeking several
//        tokens before deciding what kind of value to create).

func Unmarshal(na ipld.NodeAssembler, tokSrc shared.TokenSource, parseLinks bool) error {
	var st unmarshalState
	st.parseLinks = parseLinks
	done, err := tokSrc.Step(&st.tk[0])
	if err != nil {
		return err
	}
	if done && !st.tk[0].Type.IsValue() {
		return fmt.Errorf("unexpected eof")
	}
	return st.unmarshal(na, tokSrc)
}

type unmarshalState struct {
	tk         [4]tok.Token // mostly, only 0'th is used... but [1:4] are used during lookahead for links.
	shift      int          // how many times to slide something out of tk[1:4] instead of getting a new token.
	parseLinks bool
}

// step leaves a "new" token in tk[0],
// taking account of an shift left by linkLookahead.
// It's only necessary to use this when handling maps,
// since the situations resulting in nonzero shift are otherwise unreachable.
//
// At most, 'step' will be shifting buffered tokens for:
//   - the first map key
//   - the first map value (which will be a string)
//   - the second map key
// and so (fortunately! whew!) we can do this in a fixed amount of memory,
// since none of those states can reach a recursion.
func (st *unmarshalState) step(tokSrc shared.TokenSource) error {
	switch st.shift {
	case 0:
		_, err := tokSrc.Step(&st.tk[0])
		return err
	case 1:
		st.tk[0] = st.tk[1]
		st.shift--
		return nil
	case 2:
		st.tk[0] = st.tk[1]
		st.tk[1] = st.tk[2]
		st.shift--
		return nil
	case 3:
		st.tk[0] = st.tk[1]
		st.tk[1] = st.tk[2]
		st.tk[2] = st.tk[3]
		st.shift--
		return nil
	default:
		panic("unreachable")
	}
}

// linkLookahead is called after receiving a TMapOpen token;
// when it returns, we will have either created a link, OR
// it's not a link, and the caller should proceed to start a map
// and while using st.step to ensure the peeked tokens are handled, OR
// in case of error, the error should just rise.
// If the bool return is true, we got a link, and you should not
// continue to attempt to build a map.
func (st *unmarshalState) linkLookahead(na ipld.NodeAssembler, tokSrc shared.TokenSource) (bool, error) {
	// Peek next token.  If it's a "/" string, link is still a possibility
	_, err := tokSrc.Step(&st.tk[1])
	if err != nil {
		return false, err
	}
	if st.tk[1].Type != tok.TString {
		st.shift = 1
		return false, nil
	}
	if st.tk[1].Str != "/" {
		st.shift = 1
		return false, nil
	}
	// Peek next token.  If it's a string, link is still a possibility.
	//  We won't try to parse it as a CID until we're sure it's the only thing in the map, though.
	_, err = tokSrc.Step(&st.tk[2])
	if err != nil {
		return false, err
	}
	if st.tk[2].Type != tok.TString {
		st.shift = 2
		return false, nil
	}
	// Peek next token.  If it's map close, we've got a link!
	//  (Otherwise it had better be a string, because another map key is the
	//   only other valid transition here... but we'll leave that check to the caller.
	_, err = tokSrc.Step(&st.tk[3])
	if err != nil {
		return false, err
	}
	if st.tk[3].Type != tok.TMapClose {
		st.shift = 3
		return false, nil
	}
	// Okay, we made it -- this looks like a link.  Parse it.
	//  If it *doesn't* parse as a CID, we treat this as an error.
	elCid, err := cid.Decode(st.tk[2].Str)
	if err != nil {
		return false, err
	}
	if err := na.AssignLink(cidlink.Link{Cid: elCid}); err != nil {
		return false, err
	}
	return true, nil

}

// starts with the first token already primed.  Necessary to get recursion
//  to flow right without a peek+unpeek system.
func (st *unmarshalState) unmarshal(na ipld.NodeAssembler, tokSrc shared.TokenSource) error {
	// FUTURE: check for schema.TypedNodeBuilder that's going to parse a Link (they can slurp any token kind they want).
	switch st.tk[0].Type {
	case tok.TMapOpen:
		// dag-json has special needs: we pump a few tokens ahead to look for dag-json's "link" pattern.
		//  We can't actually call BeginMap until we're sure it's not gonna turn out to be a link.
		if st.parseLinks {
			gotLink, err := st.linkLookahead(na, tokSrc)
			if err != nil { // return in error if any token peeks failed or if structure looked like a link but failed to parse as CID.
				return err
			}
			if gotLink {
				return nil
			}
		}

		// Okay, now back to regularly scheduled map logic.
		ma, err := na.BeginMap(-1)
		if err != nil {
			return err
		}
		for {
			err := st.step(tokSrc) // shift next token into slot 0.
			if err != nil {        // return in error if next token unreadable
				return err
			}
			switch st.tk[0].Type {
			case tok.TMapClose:
				return ma.Finish()
			case tok.TString:
				// continue
			default:
				return fmt.Errorf("unexpected %s token while expecting map key", st.tk[0].Type)
			}
			mva, err := ma.AssembleEntry(st.tk[0].Str)
			if err != nil { // return in error if the key was rejected
				return err
			}
			// Do another shift so the next token is primed before we recurse.
			err = st.step(tokSrc)
			if err != nil { // return in error if next token unreadable
				return err
			}
			err = st.unmarshal(mva, tokSrc)
			if err != nil { // return in error if some part of the recursion errored
				return err
			}
		}
	case tok.TMapClose:
		return fmt.Errorf("unexpected mapClose token")
	case tok.TArrOpen:
		la, err := na.BeginList(-1)
		if err != nil {
			return err
		}
		for {
			_, err := tokSrc.Step(&st.tk[0])
			if err != nil {
				return err
			}
			switch st.tk[0].Type {
			case tok.TArrClose:
				return la.Finish()
			default:
				err := st.unmarshal(la.AssembleValue(), tokSrc)
				if err != nil { // return in error if some part of the recursion errored
					return err
				}
			}
		}
	case tok.TArrClose:
		return fmt.Errorf("unexpected arrClose token")
	case tok.TNull:
		return na.AssignNull()
	case tok.TString:
		return na.AssignString(st.tk[0].Str)
	case tok.TBytes:
		return na.AssignBytes(st.tk[0].Bytes)
	case tok.TBool:
		return na.AssignBool(st.tk[0].Bool)
	case tok.TInt:
		return na.AssignInt(st.tk[0].Int)
	case tok.TUint:
		return na.AssignInt(int64(st.tk[0].Uint)) // FIXME overflow check
	case tok.TFloat64:
		return na.AssignFloat(st.tk[0].Float64)
	default:
		panic("unreachable")
	}
}