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Added search functions in Ranges.v3 #288

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Oct 11, 2024
Merged

Added search functions in Ranges.v3 #288

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Added search functions in Ranges.v3
linxuanm Oct 6, 2024
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linxuanm Oct 8, 2024
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36 changes: 36 additions & 0 deletions lib/util/Ranges.v3
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Original file line number Diff line number Diff line change
Expand Up @@ -76,4 +76,40 @@ component Ranges {
}
return i;
}
// Performs binary search and returns the index.
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This code should follow the tab-per-indent level style of the surrounding Virgil code.

def binarySearch<T>(range: Range<T>, val: T, lt: (T, T) -> bool) -> int {
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I think we should name these three methods binarySearchEq, binarySearchLt, and binarySearchGt and comment them uniformly. See below.

var l = 0, r = range.length;
while (l < r) {
var mid = (l + r) >> 1;
var mid_val = range[mid];
if (lt(val, mid_val)) r = mid - 1;
else if (lt(mid_val, val)) l = mid + 1;
else return mid;
}
return -1;
}
// Returns the *first* element that is *not less* than `val` in a sorted range.
def lowerbound<T>(range: Range<T>, val: T, lt: (T, T) -> bool) -> int {
var start = 0;
var count = range.length;
while (count > 0) {
var step = count >> 1;
var mid = start + step;
if (lt(range[mid], val)) { start = mid + 1; count -= step + 1; }
else count = step;
}
return start;
}
// Returns the *first* element that is *greater* than `val` in a sorted range.
def upperbound<T>(range: Range<T>, val: T, lt: (T, T) -> bool) -> int {
var start = 0;
var count = range.length;
while (count > 0) {
var step = count >> 1;
var mid = start + step;
if (!lt(val, range[mid])) { start = mid + 1; count -= step + 1; }
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I think there is a subtle distinction between !lt(x, y) and lt(y, x). I think this should code should use the latter. I think it should be accompanied with a comment that illustrates, like so:

// Assumes that {range} is sorted according to `lt` and returns the first element that is *greater* than `val`.
// e in         | elem  elem  elem  elem  elem  elem  |
// lt(e, val)   | true  true  false false false false |
// lt(val, e)   | false false false true  true  true  |
//                                  ^

I think all three methods could use this comment.

else count = step;
}
return start;
}
}