Avoid intermediate slice views when reading token text

common/src/main/java/dev/cel/common/internal/BasicCodePointArray.java

@@ -58,13 +58,14 @@ public BasicCodePointArray slice(int i, int j) {
   }
 
   @Override
-  public int get(int index) {
-    checkElementIndex(index, size());
-    return codePoints()[offset() + index] & 0xffff;
+  public String substring(int i, int j) {
+    checkPositionIndexes(i, j, size());
+    return new String(codePoints(), offset() + i, j - i);
   }
 
   @Override
-  public final String toString() {
-    return new String(codePoints(), offset(), size());
+  public int get(int index) {
+    checkElementIndex(index, size());
+    return codePoints()[offset() + index] & 0xffff;
   }
 }

common/src/main/java/dev/cel/common/internal/CelCodePointArray.java

@@ -36,6 +36,14 @@ public abstract class CelCodePointArray {
   /** Returns a new {@link CelCodePointArray} that is a subview of this between [i, j). */
   public abstract CelCodePointArray slice(int i, int j);
 
+  /**
+   * Returns the code points between [i, j) as a string.
+   *
+   * <p>Equivalent to {@code slice(i, j).toString()}, but does not materialize the intermediate
+   * view. Lexing and parsing call this for every literal and identifier.
+   */
+  public abstract String substring(int i, int j);
+
   /** Get the code point at the given index. */
   public abstract int get(int index);
 
@@ -55,7 +63,9 @@ public boolean isEmpty() {
   }
 
   @Override
-  public abstract String toString();
+  public final String toString() {
+    return substring(0, size());
+  }
 
   public static CelCodePointArray fromString(String text) {
     if (isNullOrEmpty(text)) {

common/src/main/java/dev/cel/common/internal/EmptyCodePointArray.java

@@ -14,6 +14,8 @@
 
 package dev.cel.common.internal;
 
+import static com.google.common.base.Preconditions.checkPositionIndexes;
+
 import com.google.common.collect.ImmutableList;
 import com.google.errorprone.annotations.DoNotCall;
 import com.google.errorprone.annotations.Immutable;
@@ -51,6 +53,12 @@ public int get(int index) {
         String.format("index (%s) must not be greater than size (0)", index));
   }
 
+  @Override
+  public String substring(int i, int j) {
+    checkPositionIndexes(i, j, 0);
+    return "";
+  }
+
   @Override
   public int size() {
     return 0;
@@ -60,9 +68,4 @@ public int size() {
   public ImmutableList<Integer> lineOffsets() {
     return ImmutableList.of(1);
   }
-
-  @Override
-  public String toString() {
-    return "";
-  }
 }

common/src/main/java/dev/cel/common/internal/Latin1CodePointArray.java

@@ -58,13 +58,14 @@ public Latin1CodePointArray slice(int i, int j) {
   }
 
   @Override
-  public int get(int index) {
-    checkElementIndex(index, size());
-    return Byte.toUnsignedInt(codePoints()[offset() + index]);
+  public String substring(int i, int j) {
+    checkPositionIndexes(i, j, size());
+    return new String(codePoints(), offset() + i, j - i, ISO_8859_1);
   }
 
   @Override
-  public final String toString() {
-    return new String(codePoints(), offset(), size(), ISO_8859_1);
+  public int get(int index) {
+    checkElementIndex(index, size());
+    return Byte.toUnsignedInt(codePoints()[offset() + index]);
   }
 }

common/src/main/java/dev/cel/common/internal/SupplementalCodePointArray.java

@@ -59,13 +59,14 @@ public SupplementalCodePointArray slice(int i, int j) {
   }
 
   @Override
-  public int get(int index) {
-    checkElementIndex(index, size());
-    return codePoints()[offset() + index];
+  public String substring(int i, int j) {
+    checkPositionIndexes(i, j, size());
+    return new String(codePoints(), offset() + i, j - i);
   }
 
   @Override
-  public final String toString() {
-    return new String(codePoints(), offset(), size());
+  public int get(int index) {
+    checkElementIndex(index, size());
+    return codePoints()[offset() + index];
   }
 }

common/src/test/java/dev/cel/common/internal/CelCodePointArrayTest.java

@@ -15,6 +15,7 @@
 package dev.cel.common.internal;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
@@ -39,6 +40,93 @@ public void computeLineOffset(
         .inOrder();
   }
 
+  @Test
+  public void substring_empty() {
+    CelCodePointArray empty = CelCodePointArray.fromString("");
+    assertThat(empty).isInstanceOf(EmptyCodePointArray.class);
+
+    assertThat(empty.substring(0, 0)).isEmpty();
+    assertThrows(IndexOutOfBoundsException.class, () -> empty.substring(0, 1));
+    assertThrows(IndexOutOfBoundsException.class, () -> empty.substring(-1, 0));
+    assertThrows(IndexOutOfBoundsException.class, () -> empty.substring(1, 0));
+    assertThrows(IndexOutOfBoundsException.class, () -> empty.substring(1, 1));
+  }
+
+  @Test
+  public void substring_latin1() {
+    CelCodePointArray latin1 = CelCodePointArray.fromString("hello world");
+    assertThat(latin1).isInstanceOf(Latin1CodePointArray.class);
+
+    assertThat(latin1.substring(0, 5)).isEqualTo("hello");
+    assertThat(latin1.substring(6, 11)).isEqualTo("world");
+    assertThat(latin1.substring(0, 11)).isEqualTo("hello world");
+    assertThat(latin1.substring(3, 3)).isEmpty();
+
+    assertThrows(IndexOutOfBoundsException.class, () -> latin1.substring(-1, 5));
+    assertThrows(IndexOutOfBoundsException.class, () -> latin1.substring(0, 12));
+    assertThrows(IndexOutOfBoundsException.class, () -> latin1.substring(5, 4));
+
+    // Test on a sliced subview to ensure bounds are checked against size(), not the backing buffer
+    // length
+    CelCodePointArray sliced = latin1.slice(1, 4); // "ell", size = 3, buffer length = 11
+    assertThat(sliced.substring(0, 3)).isEqualTo("ell");
+    assertThat(sliced.substring(1, 2)).isEqualTo("l");
+
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(-1, 2));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(0, 4));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(2, 1));
+  }
+
+  @Test
+  public void substring_basic() {
+    CelCodePointArray basic = CelCodePointArray.fromString("abc \uff20 def");
+    assertThat(basic).isInstanceOf(BasicCodePointArray.class);
+
+    assertThat(basic.substring(0, 3)).isEqualTo("abc");
+    assertThat(basic.substring(4, 5)).isEqualTo("\uff20");
+    assertThat(basic.substring(6, 9)).isEqualTo("def");
+    assertThat(basic.substring(0, 9)).isEqualTo("abc \uff20 def");
+    assertThat(basic.substring(3, 3)).isEmpty();
+
+    assertThrows(IndexOutOfBoundsException.class, () -> basic.substring(-1, 5));
+    assertThrows(IndexOutOfBoundsException.class, () -> basic.substring(0, 10));
+    assertThrows(IndexOutOfBoundsException.class, () -> basic.substring(5, 4));
+
+    // Test on a sliced subview to ensure bounds are checked against size(), not the backing buffer
+    // length
+    CelCodePointArray sliced = basic.slice(1, 5); // "bc \uff20", size = 4, buffer length = 9
+    assertThat(sliced.substring(0, 4)).isEqualTo("bc \uff20");
+
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(-1, 2));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(0, 5));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(2, 1));
+  }
+
+  @Test
+  public void substring_supplemental() {
+    CelCodePointArray supp = CelCodePointArray.fromString(" text 가나다 😦😁😑 ");
+    assertThat(supp).isInstanceOf(SupplementalCodePointArray.class);
+
+    assertThat(supp.substring(0, 5)).isEqualTo(" text");
+    assertThat(supp.substring(10, 13)).isEqualTo("😦😁😑");
+    assertThat(supp.substring(0, supp.size())).isEqualTo(" text 가나다 😦😁😑 ");
+    assertThat(supp.substring(3, 3)).isEmpty();
+
+    assertThrows(IndexOutOfBoundsException.class, () -> supp.substring(-1, 5));
+    int greaterThanSize = supp.size() + 1;
+    assertThrows(IndexOutOfBoundsException.class, () -> supp.substring(0, greaterThanSize));
+    assertThrows(IndexOutOfBoundsException.class, () -> supp.substring(5, 4));
+
+    // Test on a sliced subview to ensure bounds are checked against size(), not the backing buffer
+    // length
+    CelCodePointArray sliced = supp.slice(1, 5); // "text", size = 4, buffer length = 15
+    assertThat(sliced.substring(0, 4)).isEqualTo("text");
+
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(-1, 2));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(0, 5));
+    assertThrows(IndexOutOfBoundsException.class, () -> sliced.substring(2, 1));
+  }
+
   @AutoValue
   abstract static class LineOffsetTestCase {
     abstract String text();

parser/src/main/java/dev/cel/parser/BUILD.bazel

@@ -118,6 +118,7 @@ java_library(
         "//common:source_location",
         "//common/ast",
         "//common/internal",
+        "@maven//:com_google_errorprone_error_prone_annotations",
         "@maven//:com_google_guava_guava",
         "@maven//:org_jspecify_jspecify",
     ],