fix: bound the context shown around a highlight to a window

A kind:9802 highlight can carry a huge `context` tag — a quote pulled from
the middle of a long article may ship several paragraphs of surrounding
text. Rendered whole, that fills the feed card with paragraphs around a
one-sentence highlight.

Trim the context in `HighlightQuote.of` to at most ~160 characters on each
side of the marked quote, snapping the cut to a whole-word boundary and
marking it with an ellipsis. The quote itself is always kept in full and
the marked range is re-based onto the trimmed text, so the in-context
marker still lands exactly on the highlighted passage. Short contexts are
left untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ApuEseGcFjUFqYoLhCuR91

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/highlights/HighlightQuote.kt

@@ -24,9 +24,11 @@ package com.vitorpamplona.amethyst.commons.model.highlights
  * What to render for a NIP-84 highlight: the passage to show, and the range inside it that
  * the user actually marked.
  *
- * When the event carries a `context` tag the whole surrounding sentence is shown with the
- * quote marked inside it. When it doesn't — or the quote can't be located in the context —
- * [text] is the quote alone and [marked] is null, meaning "mark all of it".
+ * When the event carries a `context` tag the surrounding passage is shown with the quote
+ * marked inside it, trimmed to a bounded window on each side so a quote pulled from the
+ * middle of a long article doesn't drag whole paragraphs into the feed. When it doesn't —
+ * or the quote can't be located in the context — [text] is the quote alone and [marked] is
+ * null, meaning "mark all of it".
  */
 data class HighlightQuote(
     val text: String,
@@ -49,7 +51,7 @@ data class HighlightQuote(
 
             val at = locate(context, highlight, prefix)
             return if (at != null) {
-                HighlightQuote(context, at until (at + highlight.length))
+                window(context, at, at + highlight.length)
             } else {
                 // Context that doesn't actually contain the quote is worse than no context:
                 // it would mark nothing and silently show text the user never highlighted.
@@ -57,6 +59,75 @@ data class HighlightQuote(
             }
         }
 
+        /**
+         * Most surrounding context to keep on each side of the marked quote, in characters.
+         * A highlight taken from the middle of a long article can carry the whole article in
+         * its `context` tag; without a cap the feed card would render several paragraphs around
+         * a one-sentence highlight. Just enough to frame the quote, no more.
+         */
+        private const val MAX_CONTEXT_CHARS_PER_SIDE = 160
+
+        private const val ELLIPSIS = "…"
+
+        /**
+         * Trims the context down to [MAX_CONTEXT_CHARS_PER_SIDE] on each side of the quote,
+         * snapping the cut to a whole-word boundary and marking it with an ellipsis. The quote
+         * itself ([start] until [endExclusive]) is always kept in full, and the returned
+         * [marked] range is re-based onto the trimmed text.
+         */
+        private fun window(
+            context: String,
+            start: Int,
+            endExclusive: Int,
+        ): HighlightQuote {
+            val lead = trimLead(context.substring(0, start))
+            val trail = trimTrail(context.substring(endExclusive))
+            val quote = context.substring(start, endExclusive)
+
+            // Nothing to trim on either side: the original context is short enough to show whole.
+            if (!lead.trimmed && !trail.trimmed) {
+                return HighlightQuote(context, start until endExclusive)
+            }
+
+            val prefix = if (lead.trimmed) "$ELLIPSIS " else ""
+            val suffix = if (trail.trimmed) " $ELLIPSIS" else ""
+
+            val text = prefix + lead.text + quote + trail.text + suffix
+            val markStart = prefix.length + lead.text.length
+            return HighlightQuote(text, markStart until (markStart + quote.length))
+        }
+
+        private class Side(
+            val text: String,
+            val trimmed: Boolean,
+        )
+
+        /** Keeps the tail of the leading context, starting at a whole word. */
+        private fun trimLead(text: String): Side {
+            if (text.length <= MAX_CONTEXT_CHARS_PER_SIDE) return Side(text, false)
+
+            var i = text.length - MAX_CONTEXT_CHARS_PER_SIDE
+            // Skip the partial word the budget landed inside, then the whitespace after it, so
+            // the kept text begins at the start of a whole word rather than mid-word.
+            while (i < text.length && !text[i].isWhitespace()) i++
+            while (i < text.length && text[i].isWhitespace()) i++
+            val cut = if (i >= text.length) text.length - MAX_CONTEXT_CHARS_PER_SIDE else i
+            return Side(text.substring(cut), true)
+        }
+
+        /** Keeps the head of the trailing context, ending at a whole word. */
+        private fun trimTrail(text: String): Side {
+            if (text.length <= MAX_CONTEXT_CHARS_PER_SIDE) return Side(text, false)
+
+            var i = MAX_CONTEXT_CHARS_PER_SIDE
+            // Retreat over the partial word the budget landed inside, then the whitespace before
+            // it, so the kept text ends at the end of a whole word rather than mid-word.
+            while (i > 0 && !text[i - 1].isWhitespace()) i--
+            while (i > 0 && text[i - 1].isWhitespace()) i--
+            val cut = if (i <= 0) MAX_CONTEXT_CHARS_PER_SIDE else i
+            return Side(text.substring(0, cut), true)
+        }
+
         /**
          * Finds [highlight] inside [context], preferring the occurrence whose preceding text
          * ends with [prefix]. Highlighters emit that prefix precisely so a repeated quote can

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/model/highlights/HighlightQuoteTest.kt

@@ -23,6 +23,7 @@ package com.vitorpamplona.amethyst.commons.model.highlights
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 
 class HighlightQuoteTest {
     @Test
@@ -89,4 +90,38 @@ class HighlightQuoteTest {
         assertEquals("some context", quote.text)
         assertNull(quote.marked)
     }
+
+    @Test
+    fun keepsShortContextWholeWithoutEllipsis() {
+        val quote = HighlightQuote.of("the merge happens slowly", "We think the merge happens slowly. It does.")
+
+        assertEquals("We think the merge happens slowly. It does.", quote.text)
+        assertTrue('…' !in quote.text)
+    }
+
+    @Test
+    fun trimsLongContextToAWindowAroundTheQuote() {
+        val filler = "word ".repeat(200).trim() // ~1000 chars of context on each side
+        val context = "$filler the marked quote $filler"
+        val quote = HighlightQuote.of("the marked quote", context)
+
+        // The whole quote survives and stays marked...
+        assertEquals("the marked quote", quote.text.substring(quote.marked!!))
+        // ...but the surrounding text is trimmed with an ellipsis on each side...
+        assertTrue(quote.text.startsWith("… "))
+        assertTrue(quote.text.endsWith(" …"))
+        // ...and the result is a small fraction of the original two-paragraph context.
+        assertTrue(quote.text.length < context.length / 2, "expected windowing, got ${quote.text.length} of ${context.length}")
+    }
+
+    @Test
+    fun trimmingSnapsToWholeWordsSoNoWordIsCutInHalf() {
+        val lead = "alpha bravo charlie delta echo foxtrot ".repeat(20) // long, space-separated
+        val context = "${lead}QUOTE"
+        val quote = HighlightQuote.of("QUOTE", context)
+
+        // The kept lead-in starts right after the ellipsis with a whole word, never a fragment.
+        val keptLead = quote.text.removePrefix("… ").removeSuffix("QUOTE")
+        assertTrue(keptLead.split(" ").first() in setOf("alpha", "bravo", "charlie", "delta", "echo", "foxtrot"))
+    }
 }