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Allow appending and then dropping the same number of elements from Stream #608

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Allow appending and then dropping the same number of elements from Stream #608

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7d0ed3c
throw `DropIndexOverflow` only when drop length exceeds app length
strikef Mar 14, 2025
9e39318
simplify `drop k (app k s)` to avoid erroneous C codegen when |drop| …
strikef Mar 14, 2025
67482fc
Move fix to `Temporal` and add explanatory comments
strikef Mar 27, 2025
b1ad228
Remove redundant diff
strikef Mar 27, 2025
c6e7a99
Fix indent and formatting
strikef Mar 27, 2025
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2 changes: 1 addition & 1 deletion copilot-language/src/Copilot/Language/Analyze.hs
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Expand Up @@ -188,7 +188,7 @@ analyzeAppend refStreams dstn e b f = do
-- append.
analyzeDrop :: Int -> Stream a -> IO ()
analyzeDrop k (Append xs _ _)
| k >= length xs = throw DropIndexOverflow
| k > length xs = throw DropIndexOverflow
| k > fromIntegral (maxBound :: DropIdx) = throw DropMaxViolation
| otherwise = return ()
analyzeDrop _ _ = throw DropAppliedToNonAppend
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16 changes: 9 additions & 7 deletions copilot-language/src/Copilot/Language/Operators/Temporal.hs
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@@ -1,5 +1,4 @@
-- Copyright © 2011 National Institute of Aerospace / Galois, Inc.

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@ivanperez-keera ivanperez-keera Feb 11, 2026

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This line should not be removed, since this change is not related to cleaning up spacing.

(We can remove the line in a separate PR.)

{-# LANGUAGE Safe #-}

-- | Temporal stream transformations.
Expand All @@ -11,7 +10,7 @@ module Copilot.Language.Operators.Temporal
import Copilot.Core (Typed)
import Copilot.Language.Prelude
import Copilot.Language.Stream
import Prelude ()
import Prelude ((==))

infixr 1 ++

Expand All @@ -32,8 +31,11 @@ infixr 1 ++
-- elements. For most kinds of streams, you cannot drop elements without
-- prepending an equal or greater number of elements to them first, as it
-- could result in undefined samples.
drop :: Typed a => Int -> Stream a -> Stream a
drop 0 s = s
drop _ ( Const j ) = Const j
drop i ( Drop j s ) = Drop (fromIntegral i + j) s
drop i s = Drop (fromIntegral i) s
drop :: (Typed a) => Int -> Stream a -> Stream a
drop 0 s = s
-- Along with simplifying the Stream, this also avoids the invalid C code
-- generated from append (array) and drop (indexing) when their lengths are the same
drop i ( Append a _ s ) | i == length a = s
drop _ ( Const j ) = Const j
drop i ( Drop j s ) = Drop (fromIntegral i + j) s
drop i s = Drop (fromIntegral i) s