// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package verify

import (
	"strings"
)

// FuncSig is a parsed // func signature from the assembly source.
type FuncSig struct {
	Name    string
	Params  []Param
	Results []Param
}

// Param is a single function parameter.
type Param struct {
	Name string
	Typ  string // "[]byte", "[]int32", "int", "*[32]uint16", etc.
}

// IsPointer reports whether the parameter type is a pointer or slice
// (i.e., it needs a buffer address in the argument block).
func (p Param) IsPointer() bool {
	return strings.HasPrefix(p.Typ, "[]") || strings.HasPrefix(p.Typ, "*")
}

// ParseFuncSig extracts the function signature from a "// func ..." comment.
func ParseFuncSig(comment string) (FuncSig, bool) {
	m := funcSigRe.FindStringSubmatch(strings.TrimSpace(comment))
	if m == nil {
		return FuncSig{}, false
	}
	sig := FuncSig{Name: m[1],
		Params: parseParamsExported(m[2])}
	// Results may be "(a int, b int)" or "int" or "(int, error)".
	res := strings.TrimSpace(m[3])
	res = strings.TrimPrefix(res, "(")
	res = strings.TrimSuffix(res, ")")
	if res != "" {
		sig.Results = parseParamsExported(res)
	}
	return sig, true
}

// parseParamsExported splits a parameter list like "a []byte, b []int32" or
// "dst, src []byte" into typed parameters.  Go syntax allows grouped names
// where the type at the end applies to every name in the group:// "dst, src []byte" means both dst and src are []byte.
func parseParamsExported(s string) []Param {
	s = strings.TrimSpace(s)
	if s == "" {
		return nil
	}
	fields := strings.Split(s, ",")
	// First pass: extract the type from each field (if present).
	types := make([]string, len(fields))
	for i, field := range fields {
		parts := strings.Fields(strings.TrimSpace(field))
		if len(parts) >= 2 {
			types[i] = parts[len(parts)-1]
		}
	}
	// Propagate types backward: a field without a type inherits the type from
	// the next field that has one (e.g. "dst" inherits "[]byte" from "src []byte").
	for i := range fields {
		if types[i] == "" {
			for j := i + 1; j < len(fields); j++ {
				if types[j] != "" {
					types[i] = types[j]
					break
				}
			}
		}
	}
	// Second pass: build params.
	var out []Param
	for i, field := range fields {
		field = strings.TrimSpace(field)
		if field == "" {
			continue
		}
		parts := strings.Fields(field)
		typ := types[i]
		if typ == "" {
			typ = parts[0] // unnamed: the whole field is the type
			out = append(out, Param{Typ: typ})
		} else {
			out = append(out, Param{Name: parts[0], Typ: typ})
		}
	}
	return out
}

// ExtractFuncSig extracts the // func signature for the named function
// from the assembly source.
func ExtractFuncSig(src, funcName string) (FuncSig, bool) {
	lines := strings.Split(src, "\n")
	for i, line := range lines {
		if sig, ok := ParseFuncSig(line); ok && sig.Name == funcName {
			// Verify the next non-comment line is the TEXT directive.
			for j := i + 1; j < len(lines); j++ {
				trimmed := strings.TrimSpace(lines[j])
				if trimmed == "" || strings.HasPrefix(trimmed, "//") {
					continue
				}
				if strings.HasPrefix(trimmed, "TEXT") {
					return sig, true
				}
				break
			}
		}
	}
	return FuncSig{}, false
}

// ArgLayout computes the ABI0 argument layout for a function signature.
// Returns the offset of each parameter in the argument block.
func ArgLayout(sig FuncSig) []ArgOffset {
	var offsets []ArgOffset
	off := 0
	for _, p := range sig.Params {
		size := paramSize(p.Typ)
		offsets = append(offsets, ArgOffset{Name: p.Name, Typ: p.Typ, Offset: off, Size: size, IsPtr: p.IsPointer()})
		off += size
	}
	return offsets
}

// ArgOffset describes one parameter's position in the argument block.
type ArgOffset struct {
	Name   string
	Typ    string
	Offset int
	Size   int
	IsPtr  bool
}

// paramSize returns the size in bytes of a parameter type in the ABI0 layout.
func paramSize(typ string) int {
	switch {
	case strings.HasPrefix(typ, "[]"):
		// Slice: pointer + length + capacity = 24 bytes.
		return 24
	case strings.HasPrefix(typ, "*"):
		// Pointer: 8 bytes.
		return 8
	case typ == "int", typ == "int64", typ == "uint64", typ == "uintptr":
		return 8
	case typ == "int32", typ == "uint32", typ == "float32":
		return 4
	case typ == "int16", typ == "uint16":
		return 2
	case typ == "int8", typ == "uint8", typ == "byte", typ == "bool":
		return 1
	default:
		// Default to 8 bytes for unknown types.
		return 8
	}
}