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//! A string table implementation with a tree-like encoding.
//!
//! Each entry in the table represents a string and is encoded as a list of
//! components where each component can either be
//!
//! 1. a string _value_ that contains actual UTF-8 string content,
//! 2. a string _ID_ that contains a reference to another entry, or
//! 3. a terminator tag which marks the end of a component list.
//!
//! The string _content_ of an entry is defined as the concatenation of the
//! content of its components. The content of a string value is its actual
//! UTF-8 bytes. The content of a string ID is the contents of the entry
//! it references.
//!
//! The byte-level encoding of component lists uses the structure of UTF-8 in
//! order to save space:
//!
//! - A valid UTF-8 codepoint never starts with the byte `0xFE`. We make use
//! of this fact by letting all string ID components start with this `0xFE`
//! prefix. Thus when we parse the contents of a value we know to stop if
//! we encounter this byte.
//!
//! - A valid UTF-8 string cannot contain the `0xFF` byte. Thus we can safely
//! use `0xFF` as our component list terminator.
//!
//! The sample composite string ["abc", ID(42), "def", TERMINATOR] would thus be
//! encoded as:
//!
//! ```ignore
//! ['a', 'b' , 'c', 254, 42, 0, 0, 0, 'd', 'e', 'f', 255]
//! ^^^^^^^^^^^^^^^^ ^^^
//! string ID with 0xFE prefix terminator (0xFF)
//! ```
//!
//! As you can see string IDs are encoded in little endian format.
//!
//! ----------------------------------------------------------------------------
//!
//! Each string in the table is referred to via a `StringId`. `StringId`s may
//! be generated in two ways:
//!
//! 1. Calling `StringTableBuilder::alloc()` which returns the `StringId` for
//! the allocated string.
//! 2. Calling `StringId::new_virtual()` to create a "virtual" `StringId` that
//! later can be mapped to an actual string via
//! `StringTableBuilder::map_virtual_to_concrete_string()`.
//!
//! String IDs allow you to deduplicate strings by allocating a string
//! once and then referring to it by id over and over. This is a useful trick
//! for strings which are recorded many times and it can significantly reduce
//! the size of profile trace files.
//!
//! `StringId`s are partitioned according to type:
//!
//! > [0 .. MAX_VIRTUAL_STRING_ID, METADATA_STRING_ID, .. ]
//!
//! From `0` to `MAX_VIRTUAL_STRING_ID` are the allowed values for virtual strings.
//! After `MAX_VIRTUAL_STRING_ID`, there is one string id (`METADATA_STRING_ID`)
//! which is used internally by `measureme` to record additional metadata about
//! the profiling session. After `METADATA_STRING_ID` are all other `StringId`
//! values.
use crate::file_header::{
write_file_header, FILE_MAGIC_STRINGTABLE_DATA, FILE_MAGIC_STRINGTABLE_INDEX,
};
use crate::serialization::Addr;
use crate::serialization::SerializationSink;
use std::{error::Error, sync::Arc};
/// A `StringId` is used to identify a string in the `StringTable`. It is
/// either a regular `StringId`, meaning that it contains the absolute address
/// of a string within the string table data. Or it is "virtual", which means
/// that the address it points to is resolved via the string table index data,
/// that maps virtual `StringId`s to addresses.
#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash)]
#[repr(C)]
pub struct StringId(u64);
impl StringId {
pub const INVALID: StringId = StringId(INVALID_STRING_ID);
#[inline]
pub fn new(id: impl Into<u64>) -> StringId {
StringId(id.into())
}
#[inline]
pub fn new_virtual(id: impl Into<u64>) -> StringId {
let id = id.into();
assert!(id <= MAX_USER_VIRTUAL_STRING_ID);
StringId(id)
}
#[inline]
pub fn is_virtual(self) -> bool {
self.0 <= METADATA_STRING_ID
}
#[inline]
pub fn as_u64(self) -> u64 {
self.0
}
#[inline]
pub fn from_addr(addr: Addr) -> StringId {
let id = addr.0.checked_add(FIRST_REGULAR_STRING_ID).unwrap();
StringId::new(id)
}
#[inline]
pub fn to_addr(self) -> Addr {
Addr(self.0.checked_sub(FIRST_REGULAR_STRING_ID).unwrap())
}
}
// See module-level documentation for more information on the encoding.
pub const TERMINATOR: u8 = 0xFF;
pub const STRING_REF_TAG: u8 = 0xFE;
pub const STRING_REF_ENCODED_SIZE: usize = 9;
/// The maximum id value a virtual string may be.
const MAX_USER_VIRTUAL_STRING_ID: u64 = 100_000_000;
/// The id of the profile metadata string entry.
pub const METADATA_STRING_ID: u64 = MAX_USER_VIRTUAL_STRING_ID + 1;
/// Some random string ID that we make sure cannot be generated or assigned to.
const INVALID_STRING_ID: u64 = METADATA_STRING_ID + 1;
pub const FIRST_REGULAR_STRING_ID: u64 = INVALID_STRING_ID + 1;
/// Write-only version of the string table
pub struct StringTableBuilder {
data_sink: Arc<SerializationSink>,
index_sink: Arc<SerializationSink>,
}
/// Anything that implements `SerializableString` can be written to a
/// `StringTable`.
pub trait SerializableString {
fn serialized_size(&self) -> usize;
fn serialize(&self, bytes: &mut [u8]);
}
// A single string is encoded as `[UTF-8 bytes][TERMINATOR]`
impl SerializableString for str {
#[inline]
fn serialized_size(&self) -> usize {
self.len() + // actual bytes
1 // terminator
}
#[inline]
fn serialize(&self, bytes: &mut [u8]) {
let last_byte_index = bytes.len() - 1;
bytes[0..last_byte_index].copy_from_slice(self.as_bytes());
bytes[last_byte_index] = TERMINATOR;
}
}
/// A single component of a string. Used for building composite table entries.
pub enum StringComponent<'s> {
Value(&'s str),
Ref(StringId),
}
impl<'s> StringComponent<'s> {
#[inline]
fn serialized_size(&self) -> usize {
match *self {
StringComponent::Value(s) => s.len(),
StringComponent::Ref(_) => STRING_REF_ENCODED_SIZE,
}
}
#[inline]
fn serialize<'b>(&self, bytes: &'b mut [u8]) -> &'b mut [u8] {
match *self {
StringComponent::Value(s) => {
bytes[..s.len()].copy_from_slice(s.as_bytes());
&mut bytes[s.len()..]
}
StringComponent::Ref(string_id) => {
// The code below assumes we use a 9-byte encoding for string
// refs, where the first byte is STRING_REF_TAG and the
// following 8 bytes are a little-endian u64 string ID value.
assert!(STRING_REF_ENCODED_SIZE == 9);
bytes[0] = STRING_REF_TAG;
bytes[1..9].copy_from_slice(&string_id.0.to_le_bytes());
&mut bytes[9..]
}
}
}
}
impl<'a> SerializableString for [StringComponent<'a>] {
#[inline]
fn serialized_size(&self) -> usize {
self.iter().map(|c| c.serialized_size()).sum::<usize>() + // size of components
1 // terminator
}
#[inline]
fn serialize(&self, mut bytes: &mut [u8]) {
assert!(bytes.len() == self.serialized_size());
for component in self.iter() {
bytes = component.serialize(bytes);
}
// Assert that we used the exact number of bytes we anticipated.
assert!(bytes.len() == 1);
bytes[0] = TERMINATOR;
}
}
macro_rules! impl_serializable_string_for_fixed_size {
($n:expr) => {
impl<'a> SerializableString for [StringComponent<'a>; $n] {
#[inline(always)]
fn serialized_size(&self) -> usize {
(&self[..]).serialized_size()
}
#[inline(always)]
fn serialize(&self, bytes: &mut [u8]) {
(&self[..]).serialize(bytes);
}
}
};
}
impl_serializable_string_for_fixed_size!(0);
impl_serializable_string_for_fixed_size!(1);
impl_serializable_string_for_fixed_size!(2);
impl_serializable_string_for_fixed_size!(3);
impl_serializable_string_for_fixed_size!(4);
impl_serializable_string_for_fixed_size!(5);
impl_serializable_string_for_fixed_size!(6);
impl_serializable_string_for_fixed_size!(7);
impl_serializable_string_for_fixed_size!(8);
impl_serializable_string_for_fixed_size!(9);
impl_serializable_string_for_fixed_size!(10);
impl_serializable_string_for_fixed_size!(11);
impl_serializable_string_for_fixed_size!(12);
impl_serializable_string_for_fixed_size!(13);
impl_serializable_string_for_fixed_size!(14);
impl_serializable_string_for_fixed_size!(15);
impl_serializable_string_for_fixed_size!(16);
fn serialize_index_entry(sink: &SerializationSink, id: StringId, addr: Addr) {
sink.write_atomic(16, |bytes| {
bytes[0..8].copy_from_slice(&id.0.to_le_bytes());
bytes[8..16].copy_from_slice(&addr.0.to_le_bytes());
});
}
impl StringTableBuilder {
pub fn new(
data_sink: Arc<SerializationSink>,
index_sink: Arc<SerializationSink>,
) -> Result<StringTableBuilder, Box<dyn Error + Send + Sync>> {
// The first thing in every stream we generate must be the stream header.
write_file_header(&mut data_sink.as_std_write(), FILE_MAGIC_STRINGTABLE_DATA)?;
write_file_header(&mut index_sink.as_std_write(), FILE_MAGIC_STRINGTABLE_INDEX)?;
Ok(StringTableBuilder {
data_sink,
index_sink,
})
}
/// Creates a mapping so that `virtual_id` will resolve to the contents of
/// `concrete_id` when reading the string table.
pub fn map_virtual_to_concrete_string(&self, virtual_id: StringId, concrete_id: StringId) {
// This assertion does not use `is_virtual` on purpose because that
// would also allow to overwrite `METADATA_STRING_ID`.
assert!(virtual_id.0 <= MAX_USER_VIRTUAL_STRING_ID);
serialize_index_entry(&*self.index_sink, virtual_id, concrete_id.to_addr());
}
pub fn bulk_map_virtual_to_single_concrete_string<I>(
&self,
virtual_ids: I,
concrete_id: StringId,
) where
I: Iterator<Item = StringId> + ExactSizeIterator,
{
// TODO: Index data encoding could have a special bulk mode that assigns
// multiple StringIds to the same addr, so we don't have to repeat
// the `concrete_id` over and over.
type MappingEntry = [u64; 2];
assert!(std::mem::size_of::<MappingEntry>() == 16);
let to_addr_le = concrete_id.to_addr().0.to_le();
let serialized: Vec<MappingEntry> = virtual_ids
.map(|from| {
let id = from.0;
assert!(id <= MAX_USER_VIRTUAL_STRING_ID);
[id.to_le(), to_addr_le]
})
.collect();
let num_bytes = serialized.len() * std::mem::size_of::<MappingEntry>();
let byte_ptr = serialized.as_ptr() as *const u8;
let bytes = unsafe { std::slice::from_raw_parts(byte_ptr, num_bytes) };
self.index_sink.write_bytes_atomic(bytes);
}
pub fn alloc_metadata<STR: SerializableString + ?Sized>(&self, s: &STR) {
let concrete_id = self.alloc(s);
let virtual_id = StringId(METADATA_STRING_ID);
assert!(virtual_id.is_virtual());
serialize_index_entry(&*self.index_sink, virtual_id, concrete_id.to_addr());
}
pub fn alloc<STR: SerializableString + ?Sized>(&self, s: &STR) -> StringId {
let size_in_bytes = s.serialized_size();
let addr = self.data_sink.write_atomic(size_in_bytes, |mem| {
s.serialize(mem);
});
StringId::from_addr(addr)
}
}