mirror of
https://github.com/systemed/tilemaker
synced 2025-02-21 13:24:09 +01:00
b77340c870
When I replaced #604 with #626, I botched extracting this part of the code. I had the trait, which taught kaguya how to serialize `PossiblyKnownTagValue`, but I missed updating the parameter type of `Attribute` to actually use it, so it was a no-op. This PR restores the behaviour of avoiding string copies, but now that we have protozero's data_view class, we can use that rather than our own weirdo struct.
64 lines
2.2 KiB
C++
64 lines
2.2 KiB
C++
#ifndef _POOLED_STRING_H
|
|
#define _POOLED_STRING_H
|
|
|
|
// std::string is quite general:
|
|
// - mutable
|
|
// - unlimited length
|
|
// - capacity can differ from size
|
|
// - can deallocate its dynamic memory
|
|
//
|
|
// Our use case, by contrast is immutable, bounded strings that live for the
|
|
// duration of the process.
|
|
//
|
|
// This gives us some room to have less memory overhead, especially on
|
|
// g++, whose implementation of std::string requires 32 bytes.
|
|
//
|
|
// Thus, we implement `PooledString`. It has a size of 16 bytes, and a small
|
|
// string optimization for strings <= 15 bytes. (We will separately teach
|
|
// AttributePair to encode Latin-character strings more efficiently, so that many
|
|
// strings of size 24 or less fit in 15 bytes.)
|
|
//
|
|
// If it needs to allocate memory, it does so from a shared pool. It is unable
|
|
// to free the memory once allocated.
|
|
|
|
// PooledString has one of three modes:
|
|
// - [126:127] = 00: small-string, length is in [120:125], lower 15 bytes are string
|
|
// - [126:127] = 10: pooled string, table is in bytes 1..3, offset in bytes 4..5, length in bytes 6..7
|
|
// - [126:127] = 11: pointer to std::string, pointer is in bytes 8..15
|
|
//
|
|
// Note that the pointer mode is not safe to be stored. It exists just to allow
|
|
// lookups in the AttributePair map before deciding to allocate a string.
|
|
|
|
#include <cstdint>
|
|
#include <vector>
|
|
#include <string>
|
|
#include <protozero/data_view.hpp>
|
|
|
|
namespace PooledStringNS {
|
|
class PooledString {
|
|
public:
|
|
// Create a short string or heap string, long-lived.
|
|
PooledString(const std::string& str);
|
|
|
|
|
|
// Wrap a protozero::data_view - only valid so long as the
|
|
// data_view that is pointed to is valid; call ensureStringIsOwned
|
|
// if you need to persist the string.
|
|
PooledString(const protozero::data_view* str);
|
|
size_t size() const;
|
|
bool operator<(const PooledString& other) const;
|
|
bool operator==(const PooledString& other) const;
|
|
bool operator!=(const PooledString& other) const;
|
|
std::string toString() const;
|
|
const char* data() const;
|
|
void ensureStringIsOwned();
|
|
|
|
private:
|
|
// 0..3 is index into table, 4..5 is offset, 6..7 is length
|
|
uint8_t storage[16];
|
|
};
|
|
}
|
|
|
|
using PooledString = PooledStringNS::PooledString;
|
|
|
|
#endif
|