73 lines
2.2 KiB
Rust
73 lines
2.2 KiB
Rust
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// SPDX-License-Identifier: GPL-2.0
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//! String representations.
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use core::fmt;
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/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
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///
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/// It does not fail if callers write past the end of the buffer so that they can calculate the
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/// size required to fit everything.
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///
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/// # Invariants
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///
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/// The memory region between `pos` (inclusive) and `end` (exclusive) is valid for writes if `pos`
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/// is less than `end`.
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pub(crate) struct RawFormatter {
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// Use `usize` to use `saturating_*` functions.
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#[allow(dead_code)]
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beg: usize,
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pos: usize,
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end: usize,
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}
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impl RawFormatter {
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/// Creates a new instance of [`RawFormatter`] with the given buffer pointers.
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///
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/// # Safety
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///
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/// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
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/// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
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pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
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// INVARIANT: The safety requirements guarantee the type invariants.
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Self {
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beg: pos as _,
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pos: pos as _,
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end: end as _,
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}
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}
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/// Returns the current insert position.
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///
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/// N.B. It may point to invalid memory.
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pub(crate) fn pos(&self) -> *mut u8 {
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self.pos as _
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}
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}
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impl fmt::Write for RawFormatter {
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fn write_str(&mut self, s: &str) -> fmt::Result {
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// `pos` value after writing `len` bytes. This does not have to be bounded by `end`, but we
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// don't want it to wrap around to 0.
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let pos_new = self.pos.saturating_add(s.len());
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// Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
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let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);
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if len_to_copy > 0 {
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// SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
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// yet, so it is valid for write per the type invariants.
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unsafe {
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core::ptr::copy_nonoverlapping(
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s.as_bytes().as_ptr(),
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self.pos as *mut u8,
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len_to_copy,
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)
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};
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}
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self.pos = pos_new;
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Ok(())
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}
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}
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