Implement range/slice syntax for excluding the end point

ref #292

documentation/intermediate-dsl.md

@@ -133,11 +133,6 @@ Results in `$ = [4/3, 5/4 "my third", 3/2 "fif", 2 "octave"]`. Notice how 4/3 wa
 ### Boolean conversion
 `true` is converted to `1` and `false` to `0` before pushing them onto the current scale.
 
-## Ranges
-Ranges of integers are generated by giving the start and end points e.g. `[1..5]` evaluates to `[1, 2, 3, 4, 5]`.
-
-To skip over values you can specify the second element `[1,3..10]` evaluates to `[1, 3, 5, 7, 9]` (the iteration stops and rejects after reaching `11`).
-
 ## Variables
 Variables in SonicWeave can hold any type of value. They must be declared before use. Variables declared `const` cannot be re-assigned while `let` variables may change what value they refer to.
 
@@ -683,6 +678,8 @@ To skip over values you can specify the second element `[1,3..10]` evaluates to
 
 Reversed ranges require the second element `[5..1]` evaluates to the empty array `[]` but `[5,4..1]` equals `[5, 4, 3, 2, 1]` as expected.
 
+Some times it's practical to exclude the end point e.g. `[0 .. < 3]` evaluates to `[0, 1, 2]`.
+
 ### Harmonic segments
 Segments of the (musical) harmonic series can be obtained by specifying the root and the harmonic of equivalence e.g. `4::8` evaluates to `[5/4, 6/4, 7/4, 8/4]`.
 
@@ -732,6 +729,8 @@ Range syntax inside array access gets a copy of a subset of the array e.g. `[1,
 
 In slice syntax the end points are optional e.g. `[1, 2, 3][..]` evaluates to `[1, 2, 3]` (a new copy).
 
+Excluding the end point can be handy to get the first n elements. `"Greetings!"[..<5]` evaluates to `"Greet"`.
+
 ## Records
 Record literals are constructed using `key: value` pairs inside curly brackets e.g. `{fif: 3/2, "my octave": 2/1}`.
 

src/ast.ts

@@ -253,6 +253,7 @@ export type ArraySlice = {
   object: Expression;
   start: Expression | null;
   second: Expression | null;
+  penultimate: boolean;
   end: Expression | null;
 };
 
@@ -338,6 +339,7 @@ export type Range = {
   type: 'Range';
   start: Expression;
   second?: Expression;
+  penultimate: boolean;
   end: Expression;
 };
 

src/grammars/paren-counter.pegjs

@@ -6,7 +6,7 @@
   const empty = {parens: 0, squares: 0, curlies: 0};
 }}
 
-Start = _ content: (StringLiteral / MonzoLiteral / ValLiteral / Other)|.., _| _ EOF {
+Start = _ content: Expression|.., _| _ EOF {
   const result = {parens: 0, squares: 0, curlies: 0};
   for (const counts of content) {
     result.parens += counts.parens;
@@ -16,6 +16,30 @@ Start = _ content: (StringLiteral / MonzoLiteral / ValLiteral / Other)|.., _| _
   return result;
 }
 
+Expression
+  = RangeOrSlice
+  / StringLiteral
+  / MonzoLiteral
+  / Array
+  / ValLiteral
+  / Other
+
+RangeOrSlice
+  = '[' _ Expression _ '..' _ '<'? _ Expression _ closed: ']'? {
+    if (closed) {
+      return empty;
+    }
+    return {parens: 0, squares: 1, curlies: 0};
+  }
+
+Array
+  = '[' _ Expression|.., _| _ closed: ']'? {
+    if (closed) {
+      return empty;
+    }
+    return {parens: 0, squares: 1, curlies: 0};
+  }
+
 StringLiteral
   = '"' chars: DoubleStringCharacter* '"' { return empty; }
   / "'" chars: SingleStringCharacter* "'" { return empty; }
@@ -30,7 +54,7 @@ ValLiteral
     return empty;
   }
 
-Other = (!WhiteSpace !LineTerminatorSequence !Comment SourceCharacter)+ {
+Other = (!WhiteSpace !LineTerminatorSequence !Comment !']' SourceCharacter)+ {
   const t = text();
   const parens = (t.match(/\(/g) ?? []).length - (t.match(/\)/g) ?? []).length;
   const squares = (t.match(/\[/g) ?? []).length - (t.match(/\]/g) ?? []).length;

src/grammars/sonic-weave.pegjs

@@ -799,6 +799,10 @@ LabeledCommaDecimal
     return labels.reduce(operatorReducerLite, object);
   }
 
+RangeDotsPenultimate = '..' _ penultimate: '<'? {
+  return !!penultimate;
+}
+
 CallTail
   = head: (__ '(' _ @ArgumentList _ ')')
     tail: (
@@ -808,8 +812,8 @@ CallTail
       / __ '[' _ key: Expression _ ']' {
         return { type: 'AccessExpression', key };
       }
-      / __ '[' _ start: Expression? _ second: (',' _ @Expression)? _ '..' _ end: Expression? _ ']' {
-        return { type: 'ArraySlice', start, second, end };
+      / __ '[' _ start: Expression? _ second: (',' _ @Expression)? _ penultimate: RangeDotsPenultimate _ end: Expression? _ ']' {
+        return { type: 'ArraySlice', start, second, penultimate, end };
       }
     )* {
       tail.unshift({ type: 'CallExpression', args: head});
@@ -854,9 +858,9 @@ TrueAccessExpression
   }
 
 ArraySlice
-  = head: Primary tail: (_ '[' _ @Expression? @(_ ',' _ @Expression)? _ '..' _ @Expression? _ ']')* {
-    return tail.reduce( (object, [start, second, end]) => {
-      return { type: 'ArraySlice', object, start, second, end };
+  = head: Primary tail: (_ '[' _ @Expression? @(_ ',' _ @Expression)? _ @RangeDotsPenultimate _ @Expression? _ ']')* {
+    return tail.reduce( (object, [start, second, penultimate, end]) => {
+      return { type: 'ArraySlice', object, start, second, penultimate, end };
     }, head);
   }
 
@@ -922,20 +926,22 @@ Primary
   / StringLiteral
 
 UnitStepRange
-  = '[' _ start: Expression _ '..' _ end: Expression _ ']' {
+  = '[' _ start: Expression _ penultimate: RangeDotsPenultimate _ end: Expression _ ']' {
     return {
       type: 'Range',
       start,
+      penultimate,
       end,
     };
   }
 
 StepRange
-  = '[' _ start: Expression _ ',' _ second: Expression _ '..' _ end: Expression _ ']' {
+  = '[' _ start: Expression _ ',' _ second: Expression _ penultimate: RangeDotsPenultimate _ end: Expression _ ']' {
     return {
       type: 'Range',
       start,
       second,
+      penultimate,
       end,
     };
   }

src/parser/__tests__/expression.spec.ts

@@ -540,6 +540,16 @@ describe('SonicWeave expression evaluator', () => {
     expect(ba).toBe('ba');
   });
 
+  it('supports penultimation tag in slices', () => {
+    const oba = evaluate('"foobar"[2..<5]');
+    expect(oba).toBe('oba');
+  });
+
+  it('supports penultimation tag in slices (first two)', () => {
+    const fo = evaluate('"foobar"[..<2]');
+    expect(fo).toBe('fo');
+  });
+
   it('supports indexing on str calls', () => {
     const C = evaluate('str(C4)[0]');
     expect(C).toBe('C');
@@ -616,12 +626,17 @@ describe('SonicWeave expression evaluator', () => {
     expect(stuff).toEqual(['1', ...['2', '3'], '4']);
   });
 
-  it('cannot produce empty ranges', () => {
+  it('cannot produce empty ranges without the penultimation flag', () => {
     const zero = evaluate('[0..0]') as Interval[];
     expect(zero).toHaveLength(1);
     expect(zero[0].toInteger()).toBe(0);
   });
 
+  it('can produce empty ranges with the penultimation flag', () => {
+    const zero = evaluate('[0..<0]') as Interval[];
+    expect(zero).toHaveLength(0);
+  });
+
   it('can produce empty segments', () => {
     const nothing = evaluate('1::1') as Interval[];
     expect(nothing).toHaveLength(0);

src/parser/__tests__/paren-counter.spec.ts

@@ -53,4 +53,29 @@ describe('Parenthesis counter', () => {
     const counts = parse('if(1){\n[-1 1>\n');
     expect(counts).toEqual({parens: 0, squares: 0, curlies: 1});
   });
+
+  it('works with penultimate ranges', () => {
+    const counts = parse('[1 .. < 5]');
+    expect(counts).toEqual({parens: 0, squares: 0, curlies: 0});
+  });
+
+  it('works with penultimate ranges (unclosed)', () => {
+    const counts = parse('[1 .. < 5');
+    expect(counts).toEqual({parens: 0, squares: 1, curlies: 0});
+  });
+
+  it('works with arrays', () => {
+    const counts = parse('[1, 2, "hello"]');
+    expect(counts).toEqual({parens: 0, squares: 0, curlies: 0});
+  });
+
+  it('works with arrays (unclosed)', () => {
+    const counts = parse('[1, 2, "hello"');
+    expect(counts).toEqual({parens: 0, squares: 1, curlies: 0});
+  });
+
+  it('works with subgroup tails', () => {
+    const counts = parse('[1 2>@2..');
+    expect(counts).toEqual({parens: 0, squares: 0, curlies: 0});
+  });
 });

src/parser/__tests__/sonic-weave-ast.spec.ts

@@ -221,6 +221,7 @@ describe('SonicWeave Abstract Syntax Tree parser', () => {
       expression: {
         type: 'Range',
         start: {type: 'IntegerLiteral', value: 1n},
+        penultimate: false,
         end: {type: 'IntegerLiteral', value: 10n},
       },
     });
@@ -348,10 +349,12 @@ describe('SonicWeave Abstract Syntax Tree parser', () => {
             object: {type: 'Identifier', id: 'x'},
             start: {type: 'IntegerLiteral', value: 0n},
             second: {type: 'IntegerLiteral', value: 2n},
+            penultimate: false,
             end: {type: 'IntegerLiteral', value: 10n},
           },
           start: {type: 'IntegerLiteral', value: 1n},
           second: null,
+          penultimate: false,
           end: {type: 'IntegerLiteral', value: 2n},
         },
         key: {type: 'IntegerLiteral', value: 0n},
@@ -470,6 +473,7 @@ describe('SonicWeave Abstract Syntax Tree parser', () => {
             object: {type: 'Identifier', id: 'arr'},
             start: null,
             second: null,
+            penultimate: false,
             end: null,
           },
           key: {type: 'IntegerLiteral', value: 1n},

src/parser/__tests__/stdlib.spec.ts

@@ -1478,7 +1478,7 @@ describe('SonicWeave standard library', () => {
     expect(range).toEqual(['3', '5', '7']);
   });
 
-  it('has Python 2 ranges (start + end)', () => {
+  it('has Python 2 ranges (start with end)', () => {
     const range = expand('range(-1, 3)');
     expect(range).toEqual(['-1', '0', '1', '2']);
   });
@@ -1487,4 +1487,9 @@ describe('SonicWeave standard library', () => {
     const range = expand('range(3)');
     expect(range).toEqual(['0', '1', '2']);
   });
+
+  it('has Python 2 ranges (negative step)', () => {
+    const range = expand('range(3, 0, -1)');
+    expect(range).toEqual(['3', '2', '1']);
+  });
 });

src/parser/expression.ts

@@ -852,6 +852,7 @@ export class ExpressionVisitor {
 
     let start = 0;
     let step = 1;
+    const pu = node.penultimate;
     let end = -1;
 
     if (node.start) {
@@ -887,14 +888,14 @@ export class ExpressionVisitor {
 
     if (step > 0) {
       start = Math.max(0, start);
-      if (start > end || start >= object.length) {
+      if ((pu ? start >= end : start > end) || start >= object.length) {
         return empty;
       }
       end = Math.min(object.length - 1, end);
 
       const result = [object[start]];
       let next = start + step;
-      while (next <= end) {
+      while (pu ? next < end : next <= end) {
         result.push(object[next]);
         next += step;
       }
@@ -904,14 +905,14 @@ export class ExpressionVisitor {
       return result as Interval[];
     } else if (step < 0) {
       start = Math.min(object.length - 1, start);
-      if (start < end || start < 0) {
+      if ((pu ? start <= end : start < end) || start < 0) {
         return empty;
       }
       end = Math.max(0, end);
 
       const result = [object[start]];
       let next = start + step;
-      while (next >= end) {
+      while (pu ? next > end : next >= end) {
         result.push(object[next]);
         next += step;
       }
@@ -1980,6 +1981,7 @@ export class ExpressionVisitor {
   protected visitRange(node: Range): Interval[] {
     const start = this.visit(node.start);
     const end = this.visit(node.end);
+    const pu = node.penultimate;
     if (!(start instanceof Interval && end instanceof Interval)) {
       throw new Error('Ranges must consist of intervals.');
     }
@@ -1997,23 +1999,23 @@ export class ExpressionVisitor {
       this.spendGas(Math.abs((end.valueOf() - start.valueOf()) / stepValue));
     }
     if (stepValue > 0) {
-      if (start.compare(end) > 0) {
+      if (pu ? start.compare(end) >= 0 : start.compare(end) > 0) {
         return [];
       }
       const result = [start];
       let next = start.add(step);
-      while (next.compare(end) <= 0) {
+      while (pu ? next.compare(end) < 0 : next.compare(end) <= 0) {
         result.push(next);
         next = next.add(step);
       }
       return result;
     } else if (stepValue < 0) {
-      if (start.compare(end) < 0) {
+      if (pu ? start.compare(end) <= 0 : start.compare(end) < 0) {
         return [];
       }
       const result = [start];
       let next = start.add(step);
-      while (next.compare(end) >= 0) {
+      while (pu ? next.compare(end) > 0 : next.compare(end) >= 0) {
         result.push(next);
         next = next.add(step);
       }

src/stdlib/prelude.ts

@@ -85,10 +85,7 @@ riff range(start, stop = niente, step = 1) {
     stop = start;
     start = 0;
   }
-  const result = [start, start+step .. stop];
-  if (result and result[-1] ~= stop)
-    void(pop(result));
-  return result;
+  return [start, start+step .. < stop];
 }
 
 riff sanitize(interval) {
@@ -802,7 +799,7 @@ riff repeated(times = 2, scale = $$) {
     return [];
   scale;
   const equave = scale[-1];
-  for (const level of equave ~^ [1..times-1])
+  for (const level of equave ~^ [1.. < times])
     scale ~* level;
 }
 
@@ -821,7 +818,7 @@ riff repeatedLinear(times = 2, scale = $$) {
     return [];
   scale;
   const total = (scale[-1] ~- 1);
-  for (const level of total ~* [1..times-1])
+  for (const level of total ~* [1.. < times])
     (scale ~- 1) ~+ level ~+ 1;
 }