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fusion/pointers: add pointer arithmetic operators #21

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fusion/pointers: add pointer arithmetic operators #21

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fusion/pointers: add pointer arithmetic operators
timotheecour Oct 9, 2020
941556e
fixup
timotheecour Oct 9, 2020
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1 change: 1 addition & 0 deletions changelog.md
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Original file line number Diff line number Diff line change
Expand Up @@ -19,3 +19,4 @@
- Added `jssets` module, Set for the JavaScript target
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
- Added `jsheaders` module for [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) for the JavaScript target.
- Added module `pointers` containing `toUncheckedArray` and pointer arithmetic operators and indexing
34 changes: 34 additions & 0 deletions src/fusion/pointers.nim
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Expand Up @@ -15,3 +15,37 @@ proc toUncheckedArray*[T](a: ptr T): ptr UncheckedArray[T] {.inline.} =
pa[0] += 5
doAssert a[1] == 105
cast[ptr UncheckedArray[T]](a)

template `+`*[T](p: ptr T, off: int): ptr T =
## Unsafe.
runnableExamples:
var a = @[10, 11, 12]
let pa = a[0].addr
doAssert (pa + 1)[] == 11
doAssert pa[2] == 12
pa[1] = 2
doAssert a[1] == 2
type T = typeof(p[]) # pending https://github.com/nim-lang/Nim/issues/13527
cast[ptr T](cast[ByteAddress](p) +% off * sizeof(T))
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@Araq Araq Oct 13, 2020 •
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In my own low level code I never needed the * sizeof(T) part. It's not intuitive and probably error-prone. Why try to outsmart the programmer who chose to operate on a very low level for a reason?

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see #21 (comment)


template `-`*[T](p: ptr T, off: int): ptr T =
## Unsafe.
type T = typeof(p[])
cast[ptr T](cast[ByteAddress](p) -% off * sizeof(T))

template `[]`*[T](p: ptr T, off: int): T =
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A single pointer is not an array.

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@timotheecour timotheecour Oct 13, 2020 •
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how about this; slightly more verbose than [], []= but still short enough:

# `[]` =>
template `at`*[T](p: ptr T, off: int): T = ...
# `[]=` =>
template `at=`*[T](p: ptr T, off: int, val: T) = ...

it would still allow this:

echo p.at(1)
p.at(2) = 2
p.at(2) -= 3

(at naming precedent: http://www.cplusplus.com/reference/vector/vector/at/ or https://riptutorial.com/opencv/example/6394/access-individual-pixel-values-with-cv--mat--at-t---)

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We already have the toUncheckedArray proc to offer array indexing.

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@timotheecour timotheecour Oct 16, 2020 •
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toUncheckedArray is useful but not as a replacement for that; this module is about convenience when dealing with low-level code so you use cast less often and make the intent clearer

p[1]
vs
p.toUncheckedArray[1]

That's why it's been implemented in so many places (with more or less good implementations eg many implementations have issues eg are not safe wrt multiple template argument evaluation bugs) and requested also in many places as shown here #21 (comment)

Among other things, it makes it in particular easy to adapt C/C++ code to nim with the simplest possible syntax.

## Unsafe.
(p + off)[]

template `[]=`*[T](p: ptr T, off: int, val: T) =
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A single pointer is not an array.

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see #21 (comment)

## Unsafe.
(p + off)[] = val

proc `+=`*[T](p: var ptr T, off: int) {.inline.} =
## Unsafe.
# not a template to avoid double evaluation issues
p = p + off

proc `-=`*[T](p: var ptr T, off: int) {.inline.} =
## Unsafe.
p = p - off
19 changes: 19 additions & 0 deletions tests/tpointers.nim
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@@ -0,0 +1,19 @@
import fusion/pointers

block:
var a = @[10, 11, 12]
let pa = a[0].addr
let pb = pa + 1
doAssert pb[] == 11
doAssert (pb - 1)[] == 10
pa[] = 100
doAssert a[0] == 100
doAssert pa[1] == 11

var pc = pa
pc += 1
doAssert pc[] == 11
doAssert pc[0] == 11
doAssert pc == pb
pc -= 1
doAssert pc == pa