package result

import (
	"reflect"
	"strings"
	"sync"
	"unicode"
	"unicode/utf8"

	"codeberg.org/gruf/go-byteutil"
	"codeberg.org/gruf/go-mangler"
)

// structKeys provides convience methods for a list
// of structKey field combinations used for cache keys.
type structKeys []structKey

// get fetches the structKey info for given lookup name (else, panics).
func (sk structKeys) get(name string) *structKey {
	for i := range sk {
		if sk[i].name == name {
			return &sk[i]
		}
	}
	panic("unknown lookup: \"" + name + "\"")
}

// generate will calculate and produce a slice of cache keys the given value
// can be stored under in the, as determined by receiving struct keys.
func (sk structKeys) generate(a any) []cachedKey {
	var keys []cachedKey

	// Get reflected value in order
	// to access the struct fields
	v := reflect.ValueOf(a)

	// Iteratively deref pointer value
	for v.Kind() == reflect.Pointer {
		if v.IsNil() {
			panic("nil ptr")
		}
		v = v.Elem()
	}

	// Acquire byte buffer
	buf := bufpool.Get().(*byteutil.Buffer)
	defer bufpool.Put(buf)

	for i := range sk {
		// Reset buffer
		buf.B = buf.B[:0]

		// Append each field value to buffer.
		for _, idx := range sk[i].fields {
			fv := v.Field(idx)
			fi := fv.Interface()
			buf.B = mangler.Append(buf.B, fi)
			buf.B = append(buf.B, '.')
		}

		// Drop last '.'
		buf.Truncate(1)

		// Don't generate keys for zero values
		if allowZero := sk[i].zero == ""; // nocollapse
		!allowZero && buf.String() == sk[i].zero {
			continue
		}

		// Append new cached key to slice
		keys = append(keys, cachedKey{
			key:   &sk[i],
			value: string(buf.B), // copy
		})
	}

	return keys
}

// cachedKey represents an actual cached key.
type cachedKey struct {
	// key is a reference to the structKey this
	// cacheKey is representing. This is a shared
	// reference and as such only the structKey.pkeys
	// lookup map is expecting to be modified.
	key *structKey

	// value is the actual string representing
	// this cache key for hashmap lookups.
	value string
}

// structKey represents a list of struct fields
// encompassing a single cache key, the string name
// of the lookup, the lookup map to primary cache
// keys, and the key's possible zero value string.
type structKey struct {
	// name is the provided cache lookup name for
	// this particular struct key, consisting of
	// period ('.') separated struct field names.
	name string

	// zero is the possible zero value for this key.
	// if set, this will _always_ be non-empty, as
	// the mangled cache key will never be empty.
	//
	// i.e. zero = ""  --> allow zero value keys
	//      zero != "" --> don't allow zero value keys
	zero string

	// fields is a slice of runtime struct field
	// indices, of the fields encompassed by this key.
	fields []int

	// pkeys is a lookup of stored struct key values
	// to the primary cache lookup key (int64).
	pkeys map[string]int64
}

// genStructKey will generate a structKey{} information object for user-given lookup
// key information, and the receiving generic paramter's type information. Panics on error.
func genStructKey(lk Lookup, t reflect.Type) structKey {
	var zeros []any

	// Split dot-separated lookup to get
	// the individual struct field names
	names := strings.Split(lk.Name, ".")
	if len(names) == 0 {
		panic("no key fields specified")
	}

	// Pre-allocate slice of expected length
	fields := make([]int, len(names))

	for i, name := range names {
		// Get field info for given name
		ft, ok := t.FieldByName(name)
		if !ok {
			panic("no field found for name: \"" + name + "\"")
		}

		// Check field is usable
		if !isExported(name) {
			panic("field must be exported")
		}

		// Set the runtime field index
		fields[i] = ft.Index[0]

		// Allocate new instance of field
		v := reflect.New(ft.Type)
		v = v.Elem()

		if !lk.AllowZero {
			// Append the zero value interface
			zeros = append(zeros, v.Interface())
		}
	}

	var zvalue string

	if len(zeros) > 0 {
		// Generate zero value string
		zvalue = genKey(zeros...)
	}

	return structKey{
		name:   lk.Name,
		zero:   zvalue,
		fields: fields,
		pkeys:  make(map[string]int64),
	}
}

// genKey generates a cache key for given key values.
func genKey(parts ...any) string {
	if len(parts) == 0 {
		// Panic to prevent annoying usecase
		// where user forgets to pass lookup
		// and instead only passes a key part,
		// e.g. cache.Get("key")
		// which then always returns false.
		panic("no key parts provided")
	}

	// Acquire buffer and reset
	buf := bufpool.Get().(*byteutil.Buffer)
	defer bufpool.Put(buf)
	buf.Reset()

	// Encode each key part
	for _, part := range parts {
		buf.B = mangler.Append(buf.B, part)
		buf.B = append(buf.B, '.')
	}

	// Drop last '.'
	buf.Truncate(1)

	// Return string copy
	return string(buf.B)
}

// isExported checks whether function name is exported.
func isExported(fnName string) bool {
	r, _ := utf8.DecodeRuneInString(fnName)
	return unicode.IsUpper(r)
}

// bufpool provides a memory pool of byte
// buffers use when encoding key types.
var bufpool = sync.Pool{
	New: func() any {
		return &byteutil.Buffer{B: make([]byte, 0, 512)}
	},
}