package binary

import (
	"bytes"
	"fmt"

	"github.com/tetratelabs/wabin/leb128"
	"github.com/tetratelabs/wabin/wasm"
)

var nullary = []byte{0x60, 0, 0}

// encodedOneParam is a cache of wasm.FunctionType values for param length 1 and result length 0
var encodedOneParam = map[wasm.ValueType][]byte{
	wasm.ValueTypeI32: {0x60, 1, wasm.ValueTypeI32, 0},
	wasm.ValueTypeI64: {0x60, 1, wasm.ValueTypeI64, 0},
	wasm.ValueTypeF32: {0x60, 1, wasm.ValueTypeF32, 0},
	wasm.ValueTypeF64: {0x60, 1, wasm.ValueTypeF64, 0},
}

// encodedOneResult is a cache of wasm.FunctionType values for param length 0 and result length 1
var encodedOneResult = map[wasm.ValueType][]byte{
	wasm.ValueTypeI32: {0x60, 0, 1, wasm.ValueTypeI32},
	wasm.ValueTypeI64: {0x60, 0, 1, wasm.ValueTypeI64},
	wasm.ValueTypeF32: {0x60, 0, 1, wasm.ValueTypeF32},
	wasm.ValueTypeF64: {0x60, 0, 1, wasm.ValueTypeF64},
}

// encodeFunctionType returns the wasm.FunctionType encoded in WebAssembly Binary Format.
//
// Note: Function types are encoded by the byte 0x60 followed by the respective vectors of parameter and result types.
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#function-types%E2%91%A4
func encodeFunctionType(t *wasm.FunctionType) []byte {
	paramCount, resultCount := len(t.Params), len(t.Results)
	if paramCount == 0 && resultCount == 0 {
		return nullary
	}
	if resultCount == 0 {
		if paramCount == 1 {
			if encoded, ok := encodedOneParam[t.Params[0]]; ok {
				return encoded
			}
		}
		return append(append([]byte{0x60}, encodeValTypes(t.Params)...), 0)
	} else if resultCount == 1 {
		if paramCount == 0 {
			if encoded, ok := encodedOneResult[t.Results[0]]; ok {
				return encoded
			}
		}
		return append(append([]byte{0x60}, encodeValTypes(t.Params)...), 1, t.Results[0])
	}
	// Only reached when "multi-value" is enabled because WebAssembly supports at most 1 result.
	data := append([]byte{0x60}, encodeValTypes(t.Params)...)
	return append(data, encodeValTypes(t.Results)...)
}

func decodeFunctionType(features wasm.CoreFeatures, r *bytes.Reader) (*wasm.FunctionType, error) {
	b, err := r.ReadByte()
	if err != nil {
		return nil, fmt.Errorf("read leading byte: %w", err)
	}

	if b != 0x60 {
		return nil, fmt.Errorf("%w: %#x != 0x60", ErrInvalidByte, b)
	}

	paramCount, _, err := leb128.DecodeUint32(r)
	if err != nil {
		return nil, fmt.Errorf("could not read parameter count: %w", err)
	}

	paramTypes, err := decodeValueTypes(r, paramCount)
	if err != nil {
		return nil, fmt.Errorf("could not read parameter types: %w", err)
	}

	resultCount, _, err := leb128.DecodeUint32(r)
	if err != nil {
		return nil, fmt.Errorf("could not read result count: %w", err)
	}

	// Guard >1.0 feature multi-value
	if resultCount > 1 {
		if err = features.RequireEnabled(wasm.CoreFeatureMultiValue); err != nil {
			return nil, fmt.Errorf("multiple result types invalid as %v", err)
		}
	}

	resultTypes, err := decodeValueTypes(r, resultCount)
	if err != nil {
		return nil, fmt.Errorf("could not read result types: %w", err)
	}

	ret := &wasm.FunctionType{
		Params:  paramTypes,
		Results: resultTypes,
	}
	return ret, nil
}