hakmem/core/box/tiny_header_box.h

// tiny_header_box.h - Header Box: Single Source of Truth for Header Operations
//
// Design Principles:
//   1. All header logic flows from tiny_nextptr.h specification
//   2. Encapsulates "which classes preserve headers" knowledge
//   3. Eliminates hardcoded class_idx checks (class_idx != 7, etc.)
//   4. Provides type-safe header read/write/validate operations
//
// Background:
//   - C0 (8B): next_off=0 → header overwritten by next pointer
//   - C1-C6 (16B-1024B): next_off=1 → header preserved in freelist
//   - C7 (2048B): next_off=0 → header overwritten by next pointer
//
// Migration:
//   ❌ FORBIDDEN: class_idx != 7, class_idx == 0 || class_idx == 7
//   ❌ FORBIDDEN: *(uint8_t*)base = HEADER_MAGIC | ...
//   ✅ USE: tiny_class_preserves_header(class_idx)
//   ✅ USE: tiny_header_write_if_preserved(base, class_idx)
//   ✅ USE: tiny_header_validate(base, class_idx, ...)

#ifndef TINY_HEADER_BOX_H
#define TINY_HEADER_BOX_H

#include <stdint.h>
#include <stdbool.h>
#include "../hakmem_build_flags.h"
#include "tiny_layout_box.h"
#include "../tiny_region_id.h"

// ============================================================================
// Core Predicate: Does this class preserve headers in freelist?
// ============================================================================
//
// This is the SINGLE SOURCE OF TRUTH for header preservation logic.
// All code must use this instead of hardcoded class_idx checks.
//
// Implementation:
//   - Delegates to tiny_user_offset() from tiny_layout_box.h
//   - offset=0 → header overwritten by next pointer → false
//   - offset!=0 → header preserved → true
//
// Returns:
//   true  - C1-C6: Header preserved at offset 0, next at offset 1
//   false - C0, C7: Header overwritten by next pointer at offset 0

static inline bool tiny_class_preserves_header(int class_idx) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    // Delegate to tiny_layout_box.h specification (Single Source of Truth)
    // user_offset=0 → header overwritten (C0, C7)
    // user_offset=1 → header preserved (C1-C6)
    return tiny_user_offset(class_idx) != 0;
#else
    // Headers disabled globally
    (void)class_idx;
    return false;
#endif
}

// ============================================================================
// Header Write (Conditional - for freelist/TLS SLL operations)
// ============================================================================
//
// Writes header ONLY if this class preserves headers.
// For C0/C7, writing header is pointless (next pointer will overwrite it).
//
// Use this when:
//   - Pushing blocks to TLS SLL
//   - Moving blocks from freelist to TLS SLL
//   - Splicing chains into TLS SLL
//
// DO NOT use this for:
//   - Allocation path (use tiny_header_write_for_alloc instead)

static inline void tiny_header_write_if_preserved(void* base, int class_idx) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    if (tiny_class_preserves_header(class_idx)) {
        *(uint8_t*)base = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
    }
#else
    (void)base;
    (void)class_idx;
#endif
}

// ============================================================================
// Header Validate (Conditional - for TLS SLL pop operations)
// ============================================================================
//
// Validates header ONLY if this class preserves headers.
// For C0/C7, validation is impossible (next pointer is stored at offset 0).
//
// Arguments:
//   base      - BASE pointer (not user pointer)
//   class_idx - Expected class index
//   out_got   - [optional] Store actual header byte read
//   out_expect- [optional] Store expected header byte
//
// Returns:
//   true  - Header valid OR class doesn't preserve headers (C0/C7)
//   false - Header mismatch (corruption detected)
//
// Use this when:
//   - Popping blocks from TLS SLL
//   - Validating freelist integrity

static inline bool tiny_header_validate(const void* base, int class_idx,
                                        uint8_t* out_got, uint8_t* out_expect) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    // C0/C7: Validation impossible (next pointer stored at offset 0)
    if (!tiny_class_preserves_header(class_idx)) {
        return true;  // Always valid (no header to check)
    }

    // C1-C6: Validate header
    uint8_t got = *(const uint8_t*)base;
    uint8_t expect = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);

    if (out_got) *out_got = got;
    if (out_expect) *out_expect = expect;

    return got == expect;
#else
    (void)base;
    (void)class_idx;
    (void)out_got;
    (void)out_expect;
    return true;
#endif
}

// ============================================================================
// Header Write (Unconditional - for allocation path)
// ============================================================================
//
// ALWAYS writes header, regardless of class.
// For C0/C7, header will be overwritten when block enters freelist,
// but must be valid when returned to user.
//
// Use this ONLY in allocation path:
//   - HAK_RET_ALLOC_BLOCK macro
//   - HAK_RET_ALLOC_BLOCK_TRACED macro
//   - Before returning block to user
//
// DO NOT use this for:
//   - Freelist operations (use tiny_header_write_if_preserved)
//   - TLS SLL operations (use tiny_header_write_if_preserved)

static inline void tiny_header_write_for_alloc(void* base, int class_idx) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    *(uint8_t*)base = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
#else
    (void)base;
    (void)class_idx;
#endif
}

// ============================================================================
// Header Read (for diagnostics/debugging)
// ============================================================================
//
// Reads header byte without validation.
// Returns -1 if headers disabled or class doesn't preserve headers.
//
// Use this for:
//   - Diagnostics
//   - Debug logging
//   - Corruption analysis
//
// DO NOT use this for:
//   - Validation (use tiny_header_validate)

static inline int tiny_header_read(const void* base, int class_idx) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    if (!tiny_class_preserves_header(class_idx)) {
        return -1;  // No header to read
    }
    return (int)(*(const uint8_t*)base);
#else
    (void)base;
    (void)class_idx;
    return -1;
#endif
}

#endif // TINY_HEADER_BOX_H