// Package config defines configuration types and validation for the go-kv storage engine.
package config

import (
	"fmt"
	"math"
	"time"
)

// WAL format constants derived from the binary layout specification.
const (
	walFileHeaderSize        uint64 = 32
	physicalRecordHeaderSize uint64 = 7
	walBatchHeaderSize       uint64 = 18 // flags(2) + baseSequence(8) + entryCount(4) + entriesSize(4)
)

// WalConfig holds configuration for the Write-Ahead Log subsystem.
// Zero-value WalConfig is valid and uses defaults; call Validate() to apply
// defaults and verify invariants.
type WalConfig struct {
	// MaxSegmentSize is the maximum size of a single WAL segment file in bytes.
	// Default: 64MB. Must be large enough to hold the largest possible WAL Batch.
	MaxSegmentSize uint64

	// BlockSize is the WAL block size in bytes.
	// Default: 32KB.
	BlockSize uint32

	// SyncMode controls when WAL writes are flushed to disk.
	// Phase 1 only supports "always".
	SyncMode string

	// MaxBatchEntries is the maximum number of entries in a single WAL Batch.
	// Default: 10000.
	MaxBatchEntries uint32

	// MaxBatchSize is the maximum total size of WAL Batch entries in bytes.
	// Default: 4MB.
	MaxBatchSize uint32

	// GroupCommitDelay is the maximum time to collect requests before writing a
	// WAL group commit batch. Default: 500µs. Must be > 0 and < 10ms.
	GroupCommitDelay time.Duration

	// MaxKeyBytes is the maximum size of a single key in bytes.
	// Default: 4KB.
	MaxKeyBytes uint32

	// MaxInlineValue is the maximum size of an inline value in bytes.
	// Values larger than this must use ValueLogPointer.
	// Default: 4KB.
	MaxInlineValue uint32

	// MemTableSize is the target MemTable size in bytes before triggering flush.
	// Default: 64MB.
	MemTableSize uint32

	// MaxImmutableCount is the maximum number of immutable MemTables allowed
	// before writes are stalled. Default: 3.
	MaxImmutableCount int
}

// Defaults returns a WalConfig populated with production defaults.
func Defaults() WalConfig {
	return WalConfig{
		MaxSegmentSize:    64 * 1024 * 1024, // 64MB
		BlockSize:         32 * 1024,        // 32KB
		SyncMode:          "always",
		MaxBatchEntries:   10000,
		MaxBatchSize:      4 * 1024 * 1024, // 4MB
		GroupCommitDelay:  500 * time.Microsecond,
		MaxKeyBytes:       4 * 1024,         // 4KB
		MaxInlineValue:    4 * 1024,         // 4KB
		MemTableSize:      64 * 1024 * 1024, // 64MB
		MaxImmutableCount: 3,
	}
}

// applyDefaults fills zero-valued fields with production defaults.
func (c *WalConfig) applyDefaults() {
	d := Defaults()
	if c.MaxSegmentSize == 0 {
		c.MaxSegmentSize = d.MaxSegmentSize
	}
	if c.BlockSize == 0 {
		c.BlockSize = d.BlockSize
	}
	if c.SyncMode == "" {
		c.SyncMode = d.SyncMode
	}
	if c.MaxBatchEntries == 0 {
		c.MaxBatchEntries = d.MaxBatchEntries
	}
	if c.MaxBatchSize == 0 {
		c.MaxBatchSize = d.MaxBatchSize
	}
	if c.GroupCommitDelay == 0 {
		c.GroupCommitDelay = d.GroupCommitDelay
	}
	if c.MaxKeyBytes == 0 {
		c.MaxKeyBytes = d.MaxKeyBytes
	}
	if c.MaxInlineValue == 0 {
		c.MaxInlineValue = d.MaxInlineValue
	}
	if c.MemTableSize == 0 {
		c.MemTableSize = d.MemTableSize
	}
	if c.MaxImmutableCount == 0 {
		c.MaxImmutableCount = d.MaxImmutableCount
	}
}

// Validate applies defaults and verifies that all configuration invariants hold.
// The key invariant ensures that the largest possible WAL Batch can fit into
// an empty WAL segment:
//
//	maxWalSegmentPayload >= maxEncodedWalBatchSize + worstCasePhysicalRecordOverhead + worstCaseBlockPadding
//
// All arithmetic is checked for overflow.
func (c *WalConfig) Validate() error {
	c.applyDefaults()

	if c.SyncMode != "always" {
		return fmt.Errorf("config: SyncMode %q not supported (Phase 1: only \"always\")", c.SyncMode)
	}

	if c.MaxImmutableCount < 1 {
		return fmt.Errorf("config: MaxImmutableCount must be >= 1, got %d", c.MaxImmutableCount)
	}

	if c.GroupCommitDelay <= 0 || c.GroupCommitDelay >= 10*time.Millisecond {
		return fmt.Errorf("config: GroupCommitDelay must be > 0 and < 10ms, got %s", c.GroupCommitDelay)
	}

	// --- Checked arithmetic invariant validation ---
	// Mirrors the derivation in docs/design.md § WAL Segment Rotation.

	blockSize := uint64(c.BlockSize)
	prHeaderSize := physicalRecordHeaderSize
	batchHeaderSize := walBatchHeaderSize
	maxBatchEntriesSize := uint64(c.MaxBatchSize)

	// maxEncodedWalBatchSize = batchHeaderSize + maxBatchEntriesSize
	maxEncodedWalBatchSize, err := safeAdd(batchHeaderSize, maxBatchEntriesSize)
	if err != nil {
		return fmt.Errorf("config: WAL batch size overflow: %w", err)
	}

	// maxPhysicalRecordPayload = blockSize - prHeaderSize
	if blockSize <= prHeaderSize {
		return fmt.Errorf("config: BlockSize %d must be > physical record header size %d", blockSize, prHeaderSize)
	}
	maxPhysicalRecordPayload := blockSize - prHeaderSize

	// maxPhysicalRecordCount = ceil(maxEncodedWalBatchSize / maxPhysicalRecordPayload)
	maxPhysicalRecordCount := divCeil(maxEncodedWalBatchSize, maxPhysicalRecordPayload)

	// worstCasePhysicalRecordOverhead = maxPhysicalRecordCount * prHeaderSize
	worstCasePhysicalRecordOverhead, err := safeMul(maxPhysicalRecordCount, prHeaderSize)
	if err != nil {
		return fmt.Errorf("config: physical record overhead overflow: %w", err)
	}

	// worstCaseBlockPadding = blockSize - 1 (at most one partial block of padding)
	// From design doc: worstCaseBlockPadding = 7 bytes with default block size.
	// Generalized: blockSize - maxPhysicalRecordPayload = prHeaderSize
	worstCaseBlockPadding := prHeaderSize

	// minWalSegmentPayload = maxEncodedWalBatchSize + worstCasePhysicalRecordOverhead + worstCaseBlockPadding
	partial, err := safeAdd(maxEncodedWalBatchSize, worstCasePhysicalRecordOverhead)
	if err != nil {
		return fmt.Errorf("config: segment payload calculation overflow: %w", err)
	}
	minWalSegmentPayload, err := safeAdd(partial, worstCaseBlockPadding)
	if err != nil {
		return fmt.Errorf("config: segment payload calculation overflow: %w", err)
	}

	// maxWalSegmentPayload = MaxSegmentSize - walFileHeaderSize
	if c.MaxSegmentSize <= walFileHeaderSize {
		return fmt.Errorf("config: MaxSegmentSize %d must be > WAL file header size %d",
			c.MaxSegmentSize, walFileHeaderSize)
	}
	maxWalSegmentPayload := c.MaxSegmentSize - walFileHeaderSize

	if maxWalSegmentPayload < minWalSegmentPayload {
		return fmt.Errorf("config: MaxSegmentSize %d too small: "+
			"segment payload (%d) < minimum required (%d); "+
			"need MaxSegmentSize >= %d",
			c.MaxSegmentSize, maxWalSegmentPayload, minWalSegmentPayload,
			minWalSegmentPayload+walFileHeaderSize)
	}

	return nil
}

// safeAdd returns a + b or an error if the result overflows uint64.
func safeAdd(a, b uint64) (uint64, error) {
	if a > math.MaxUint64-b {
		return 0, fmt.Errorf("uint64 overflow: %d + %d", a, b)
	}
	return a + b, nil
}

// safeMul returns a * b or an error if the result overflows uint64.
func safeMul(a, b uint64) (uint64, error) {
	if a != 0 && b > math.MaxUint64/a {
		return 0, fmt.Errorf("uint64 overflow: %d * %d", a, b)
	}
	return a * b, nil
}

// divCeil returns ceil(a / b) for b > 0.
func divCeil(a, b uint64) uint64 {
	return (a + b - 1) / b
}