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Use a binary search to find the insertion index for new elements managed by the focus zone #176

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Merged
colebemis merged 2 commits into from
Feb 6, 2023
Merged

Use a binary search to find the insertion index for new elements managed by the focus zone #176

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5edc1c5
Use a binary search to find the insertion index for new elements mana…
jbrown1618 Feb 1, 2023
5e74867
Create .changeset/gorgeous-zebras-hammer.md
jbrown1618 Feb 2, 2023
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8 changes: 8 additions & 0 deletions .changeset/gorgeous-zebras-hammer.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
---
"@primer/behaviors": patch
---

Use a binary search to find the insertion index for new elements managed by the focus zone.
For a use case with 1000 elements managed by the focus zone, added one at a time (by react),
this takes us from 500,000 calls to `compareDocumentPosition` over 1000ms to 8,000 calls
over 16ms.
38 changes: 33 additions & 5 deletions src/focus-zone.ts
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Expand Up @@ -433,11 +433,8 @@ export function focusZone(container: HTMLElement, settings?: FocusZoneSettings):
if (filteredElements.length === 0) {
return
}
// Insert all elements atomically. Assume that all passed elements are well-ordered.
const insertIndex = focusableElements.findIndex(
e => (e.compareDocumentPosition(filteredElements[0]) & Node.DOCUMENT_POSITION_PRECEDING) > 0
)
focusableElements.splice(insertIndex === -1 ? focusableElements.length : insertIndex, 0, ...filteredElements)
// Insert all elements atomically.
focusableElements.splice(findInsertionIndex(filteredElements), 0, ...filteredElements)
for (const element of filteredElements) {
// Set tabindex="-1" on all tabbable elements, but save the original
// value in case we need to disable the behavior
Expand All @@ -452,6 +449,37 @@ export function focusZone(container: HTMLElement, settings?: FocusZoneSettings):
}
}

function findInsertionIndex(elementsToInsert: HTMLElement[]) {
// Assume that all passed elements are well-ordered.
const firstElementToInsert = elementsToInsert[0]

if (focusableElements.length === 0) return 0

// Because the focusable elements are in document order,
// we can do a binary search to find the insertion index.
let iMin = 0
let iMax = focusableElements.length - 1
while (iMin <= iMax) {
const i = Math.floor((iMin + iMax) / 2)
const element = focusableElements[i]

if (followsInDocument(firstElementToInsert, element)) {
iMax = i - 1
} else {
iMin = i + 1
}
}

return iMin
}

/**
* @returns true if the second argument follows the first argument in the document
*/
function followsInDocument(first: HTMLElement, second: HTMLElement) {
return (second.compareDocumentPosition(first) & Node.DOCUMENT_POSITION_PRECEDING) > 0
}

function endFocusManagement(...elements: HTMLElement[]) {
for (const element of elements) {
const focusableElementIndex = focusableElements.indexOf(element)
Expand Down