// Package page provides transport-neutral keyset pagination for Bob SELECT queries.
// Callers own authorization, non-null stored sort values, and correspondence between
// Sort columns and the typed row key. The final column must itself be unique in the
// selected relation. Mixed directions are supported; null sort values are not.
//
// Sort IDs version the expressions, directions, key types and comparison semantics.
// Cursors expose values and allow edits; they are seek boundaries, not credentials.
// Concurrent writes or changed filters can skip or repeat rows between requests.
// Code generation may eventually align Sort and key; handwritten pairs have no
// compile-time field/type correspondence guarantee.
//
// Keys support exactly int, int8, int16, int32, int64, string, time.Time and the
// standard uuid.UUID. Named types, unsigned values, pointers and null wrappers
// require explicit caller conversion. Timestamps normalize to UTC with nanosecond
// precision. Versioned JSON cursors use unpadded base64url, limited to 16,384 bytes
// after encoding; unsupported values and malformed cursors return typed errors.
package page

import (
	"fmt"
	"slices"
	"unicode/utf8"

	"github.com/stephenafamo/bob"
)

// Order is a column's canonical ordering direction.
type Order uint8

const (
	// Asc orders smaller values first.
	Asc Order = iota
	// Desc orders larger values first.
	Desc
)

// Column declares a trusted, non-null SQL expression and its canonical direction.
// Unique asserts that this column alone is unique, including across joins.
type Column struct {
	Expression bob.Expression
	Direction  Order
	Unique     bool
}

// Sort is an immutable, caller-owned ordering contract. Change ID when its
// expressions, order, key types, uniqueness or comparison semantics change.
type Sort struct {
	ID      string
	Columns []Column
}

// Direction selects which side of an optional anchor to fetch.
type Direction uint8

const (
	// Forward maps to first/after and is the default direction.
	Forward Direction = iota
	// Backward maps to last/before; returned items retain canonical sort order.
	Backward
)

// Request carries a transport-independent page request. Zero Size uses the
// configured default; excessive sizes clamp to Maximum. Empty Cursor has no anchor.
type Request struct {
	Direction Direction
	Size      int
	Cursor    string
}

// Limits configures page sizes; require 1 <= Default <= Maximum < max int.
type Limits struct {
	Default int
	Maximum int
}

// KeyError identifies a programmer error in the row key's arity.
type KeyError struct {
	Expected int
	Actual   int
}

// Error implements error with the expected and actual key lengths.
func (e *KeyError) Error() string {
	return fmt.Sprintf("page: key length %d, expected %d", e.Actual, e.Expected)
}

// Page contains canonical-order items and cursors for their first and last rows.
// Empty pages have empty cursors. Overflow determines the fetch-direction flag;
// anchor presence determines the opposite flag, even for empty/stale results.
// The opposite flag does not prove another row exists.
type Page[T any] struct {
	Items       []T
	StartCursor string
	EndCursor   string
	HasNext     bool
	HasPrevious bool
}

// Prepared holds validated pagination and the typed row key. Construct with Prepare.
// Treat Sort expressions and the key's captured state as immutable while in use.
type Prepared[T any] struct {
	sort      Sort
	key       func(T) []any
	direction Direction
	size      int
	anchor    []any
}

// Prepare validates a request before a Bob mod can be applied. key must return
// exactly one supported concrete value per Sort column, in the same order, using
// actual stored/scanned values (including database timestamp precision).
func Prepare[T any](sort Sort, key func(T) []any, request Request, limits Limits) (*Prepared[T], error) {
	if sort.ID == "" || !utf8.ValidString(sort.ID) || len(sort.Columns) == 0 {
		return nil, fmt.Errorf("page: sort needs a valid ID and columns")
	}
	for i, c := range sort.Columns {
		if c.Expression == nil || c.Direction > Desc {
			return nil, fmt.Errorf("page: invalid sort column %d", i)
		}
	}
	if !sort.Columns[len(sort.Columns)-1].Unique {
		return nil, fmt.Errorf("page: final sort column must be unique")
	}
	if key == nil {
		return nil, fmt.Errorf("page: nil key function")
	}
	if limits.Default < 1 || limits.Maximum < limits.Default || limits.Maximum == int(^uint(0)>>1) {
		return nil, fmt.Errorf("page: invalid limits")
	}
	if request.Direction > Backward || request.Size < 0 {
		return nil, fmt.Errorf("page: invalid request direction or size")
	}
	size := request.Size
	if size == 0 {
		size = limits.Default
	}
	size = min(size, limits.Maximum)
	sort.Columns = slices.Clone(sort.Columns)
	p := &Prepared[T]{sort: sort, key: key, direction: request.Direction, size: size}
	if request.Cursor != "" {
		var err error
		p.anchor, err = decodeCursor(request.Cursor, sort)
		if err != nil {
			return nil, err
		}
	}
	return p, nil
}

// Page validates all fetched rows, including overflow, and assembles a page.
// Pass rows in query order, with at most size+1 rows. It copies retained items;
// backward pagination never reverses the caller's slice. Errors return no page.
func (p *Prepared[T]) Page(rows []T) (Page[T], error) {
	var zero Page[T]
	if len(rows) > p.size+1 {
		return zero, fmt.Errorf("page: too many fetched rows")
	}
	cursors := make([]string, len(rows))
	for i, row := range rows {
		key := p.key(row)
		if len(key) != len(p.sort.Columns) {
			return zero, &KeyError{Expected: len(p.sort.Columns), Actual: len(key)}
		}
		var err error
		cursors[i], err = encodeCursor(p.sort.ID, key)
		if err != nil {
			return zero, err
		}
	}
	overflow := len(rows) > p.size
	n := min(len(rows), p.size)
	result := Page[T]{Items: slices.Clone(rows[:n])}
	if n > 0 {
		result.StartCursor, result.EndCursor = cursors[0], cursors[n-1]
	}
	if p.direction == Backward {
		slices.Reverse(result.Items)
		result.StartCursor, result.EndCursor = result.EndCursor, result.StartCursor
		result.HasPrevious, result.HasNext = overflow, len(p.anchor) > 0
	} else {
		result.HasNext, result.HasPrevious = overflow, len(p.anchor) > 0
	}
	return result, nil
}