refactor to only have one context per view

pkg/gui/patch_exploring/focus.go

@@ -0,0 +1,53 @@
+package patch_exploring
+
+import "github.com/jesseduffield/lazygit/pkg/utils"
+
+func calculateOrigin(currentOrigin int, bufferHeight int, firstLineIdx int, lastLineIdx int, selectedLineIdx int, mode selectMode) int {
+	needToSeeIdx, wantToSeeIdx := getNeedAndWantLineIdx(firstLineIdx, lastLineIdx, selectedLineIdx, mode)
+
+	return calculateNewOriginWithNeededAndWantedIdx(currentOrigin, bufferHeight, needToSeeIdx, wantToSeeIdx)
+}
+
+// we want to scroll our origin so that the index we need to see is in view
+// and the other index we want to see (e.g. the other side of a line range)
+// is in as close to being in view as possible.
+func calculateNewOriginWithNeededAndWantedIdx(currentOrigin int, bufferHeight int, needToSeeIdx int, wantToSeeIdx int) int {
+	origin := currentOrigin
+	if needToSeeIdx < currentOrigin {
+		origin = needToSeeIdx
+	} else if needToSeeIdx > currentOrigin+bufferHeight {
+		origin = needToSeeIdx - bufferHeight
+	}
+
+	bottom := origin + bufferHeight
+
+	if wantToSeeIdx < origin {
+		requiredChange := origin - wantToSeeIdx
+		allowedChange := bottom - needToSeeIdx
+		return origin - utils.Min(requiredChange, allowedChange)
+	} else if wantToSeeIdx > origin+bufferHeight {
+		requiredChange := wantToSeeIdx - bottom
+		allowedChange := needToSeeIdx - origin
+		return origin + utils.Min(requiredChange, allowedChange)
+	} else {
+		return origin
+	}
+}
+
+func getNeedAndWantLineIdx(firstLineIdx int, lastLineIdx int, selectedLineIdx int, mode selectMode) (int, int) {
+	switch mode {
+	case LINE:
+		return selectedLineIdx, selectedLineIdx
+	case RANGE:
+		if selectedLineIdx == firstLineIdx {
+			return firstLineIdx, lastLineIdx
+		} else {
+			return lastLineIdx, firstLineIdx
+		}
+	case HUNK:
+		return firstLineIdx, lastLineIdx
+	default:
+		// we should never land here
+		panic("unknown mode")
+	}
+}