Arrow termination

[Akshay-2007-1]

Description

Closes #3789.
Closes #3755
Closes #3795

Root cause in ArrowFromText.calculateSteps() (else branch):

The else branch handles arrows where the target (to.x() >= from.x()) is to the right of the text. It computes:
preTerminalX = max(frameExitX, to.x() - terminalSegmentLength)

When frameExitX (the frame's right edge + 20px) extends past the closure's left edge, which happens when the closure is positioned within or just past the frame boundary — preTerminalX > to.x(). The arrow then travels right past the closure, turns down, and comes back LEFT to land at to.x() (the left edge), visually piercing through the closure.

Fix: detect when preTerminalX > to.x() (approach is from the right) and land at to.x() + to.width() (the right edge) instead. The two previous fixes to ArrowFromArrayUnit remain correct for their own scenario.

Before	After

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Before	After

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Before	After

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Code quality improvements

How to test

Look at the following code from #3789 and others!

function add_streams(s1, s2) {
return pair(head(s1) + head(s2),
() => add_streams(stream_tail(s1), stream_tail(s2)));
}
const strm = pair(1, () => pair(2, () => add_streams(strm, strm)));
eval_stream(strm, 10);

---

const y = 2;
const f = () => null;
const q = pair(y, f);

{
    const x = 1;
    const p = pair(x, 2);
    set_tail(p, q);
}

---

function fun(x) {
    return y => x;
}

const someName = fun(1);

Checklist

• I have tested this code
• I have updated the documentation

[gemini-code-assist]

Code Review

This pull request improves arrow routing logic in the CSE machine. Specifically, it updates ArrowFromArrayUnit to dynamically calculate the target X coordinate based on the source center, and updates ArrowFromText to handle cases where the arrow approaches from the right. A review comment points out an edge case in ArrowFromText where the arrow could pierce the target if frameExitX falls inside it, and provides a code suggestion to safely position preTerminalX to the right of the target.

[RichDom2185]

@coderabbitai review

[coderabbitai]

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Review triggered.

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[coderabbitai]

Walkthrough

Two arrow routing calculations in the CSE Machine visualization are refined to compute corrected terminal X coordinates. ArrowFromArrayUnit now selects target X based on source-center spatial relationships to function values, and ArrowFromText chooses the terminal X coordinate more carefully for non-left-bending arrow paths, addressing cases where arrows previously pointed to the wrong edge of function objects.

Changes

Arrow Endpoint Selection Corrections

Layer / File(s)	Summary
Array unit arrow endpoint selection for function values src/features/cseMachine/components/arrows/ArrowFromArrayUnit.tsx	ArrowFromArrayUnit.calculateSteps now derives sourceCenterX and selects targetX by comparing the source center against the target's bounds, choosing the target's left or right edge for out-of-range cases or the target center when within range.
Text arrow endpoint selection for non-left-bending routes src/features/cseMachine/components/arrows/ArrowFromText.tsx	ArrowFromText.calculateSteps introduces terminalTargetX logic for non-left-bending routing, choosing the target's right edge when preTerminalX overshoots the target's left edge, otherwise using the target's original X position.

🎯 2 (Simple) | ⏱️ ~8 minutes

🐰 Two arrows now find their way,
No longer lost or led astray,
They calculate with thoughtful care,
Where source and target truly pair,
Geometry bends to logic's play! 🎯

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Title check	❓ Inconclusive	The title 'Arrow termination' is vague and generic, providing no specific details about which component or aspect of arrow routing is being fixed.	Use a more specific title like 'Fix arrow termination for right-approaching closures' to clearly indicate the nature and scope of the fix.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR correctly addresses issue #3789 by fixing arrow termination for right-approaching closures, detecting when preTerminalX > to.x() and landing at the right edge instead of left edge.
Out of Scope Changes check	✅ Passed	Both file changes (ArrowFromArrayUnit and ArrowFromText) are directly related to fixing arrow termination issues as specified in issue #3789, with no unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description check	✅ Passed	The pull request description is comprehensive and follows the template structure. It includes a clear summary with issue references, root cause analysis, the implemented fix with visual before/after comparisons, specifies the change type as a bug fix, and provides test instructions with code examples.

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