[libc-commits] [libc] [libc] Add regex AST and ExprPool (PR #198728)

[Alexey Samsonov via libc-commits]

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+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// Pool for Regular Expression AST nodes (Class Definitions).
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_REGEX_REGEX_EXPR_POOL_H
+#define LLVM_LIBC_SRC___SUPPORT_REGEX_REGEX_EXPR_POOL_H
+
+#include "src/__support/CPP/expected.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/regex/regex_ast.h"
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+/// An arena-based pool for Regular Expression AST nodes.
+///
+/// This class manages the allocation and hash-consing of Expr nodes. All
+/// nodes created through this pool are owned by it and will be freed when
+/// the pool is destroyed. Hash-consing ensures that identical expressions
+/// are represented by the same pointer, enabling fast comparison and
+/// derivative normalization.
+class ExprPool {
+  /// Internal storage block for AST nodes.
+  ///
+  /// Blocks are allocated on demand to avoid large contiguous allocations
+  /// and are linked together in a list for cleanup.
+  /// TODO: Consider adopting cpp::forward_list for block management once
+  /// it is available in LLVM-libc.
+  struct Block {
+    /// Number of Expr nodes stored in each block.
+    static constexpr size_t BLOCK_SIZE = 256;
+    /// The actual storage for Expr nodes.
+    Expr nodes[BLOCK_SIZE];
+    /// Pointer to the next block in the chain.
+    Block *next = nullptr;
+    /// Number of nodes currently used in this block.
+    size_t used = 0;
+
+    /// Initialises an empty block.
+    Block();
+  };
+
+  /// The first block in the allocation chain.
+  Block *head = nullptr;
+  /// The block currently being used for new node allocations.
+  Block *current = nullptr;
+  /// Total number of nodes allocated across all blocks.
+  size_t node_count = 0;
+
+  /// The size of the hash table used for hash-consing (interning) expression
+  /// nodes. Choosing 0x4000 (16,384) is the smallest power of two that keeps
+  /// the load factor below 70% when the pool reaches its limit of 10,000 nodes
+  /// (peak load factor is ~61%). Using a power of two allows the compiler to
+  /// optimize the modulo indexing into an efficient bitwise AND, while the low
+  /// load factor minimizes collisions and guarantees O(1) average interning
+  /// time.
+  static constexpr size_t HASH_TABLE_SIZE = 0x4000;
+  /// Hash table storing pointers to unique Expr nodes.
+  Expr **hashtable = nullptr;
+
+  /// Core hash-consing function (Interning).
+  ///
+  /// Guarantees that for any two identical structural definitions of an Expr,
+  /// this function will return the same pointer. This enables O(1) structural
+  /// equality via pointer comparison.
+  ///
+  /// \param e A structural definition (proto-node) to intern.
+  /// \returns A pointer to the unique, stable instance in the arena,
+  ///          or REG_ESPACE on failure.
+  cpp::expected<Expr *, int> intern(const Expr &e);
+
+  /// The maximum number of nodes allowed in the pool to prevent memory
----------------
vonosmas wrote:

Nit: moved next to `HASH_TABLE_SIZE` above and deduplicate comments.

https://github.com/llvm/llvm-project/pull/198728