Add ljust and rjust methods to str built-in type

docs/lexicon.html

@@ -411,6 +411,32 @@ <h3 class="title-3">
           - Replaces named parameters in the string.
         </span>
       </li>
+      <li>
+        <span>
+          <code class="code"
+            ><span class="fn-name">ljust</span><span class="fn-p">(</span
+            ><span class="fn-arg">width</span>[,
+            <span class="fn-arg">fillchar</span>]<span class="fn-p">)</span></code
+          >
+          - returns a copy of this string left-padded with the character given
+          in <code class="code">fillchar</code> (or the default <code class="code"
+          >' '</code> if no value is given) until it is  <code class="code"
+          >width</code> characters long.
+        </span>
+      </li>
+      <li>
+        <span>
+          <code class="code"
+            ><span class="fn-name">rjust</span><span class="fn-p">(</span
+            ><span class="fn-arg">width</span>[,
+            <span class="fn-arg">fillchar</span>]<span class="fn-p">)</span></code
+          >
+          - returns a copy of this string right-padded with the character given
+          in <code class="code">fillchar</code> (or the default <code class="code"
+          >' '</code> if no value is given) until it is  <code class="code"
+          >width</code> characters long.
+        </span>
+      </li>
       <li>
         <span>
           <code class="code"

rules/builtins.build_defs

@@ -89,6 +89,10 @@ def endswith(self:str, s:str) -> bool:
     pass
 def format(self:str) -> str:
     pass
+def ljust(self:str, width:int, fillchar:str=' ') -> str:
+    pass
+def rjust(self:str, width:int, fillchar:str=' ') -> str:
+    pass
 def lstrip(self:str, cutset:str=' \n') -> str:
     pass
 def rstrip(self:str, cutset:str=' \n') -> str:

src/parse/asp/builtins.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"sync"
 	"unicode"
+	"unicode/utf8"
 
 	"github.com/Masterminds/semver/v3"
 	"github.com/manifoldco/promptui"
@@ -88,6 +89,8 @@ func registerBuiltins(s *scope) {
 		"rpartition":   setNativeCode(s, "rpartition", strRPartition),
 		"startswith":   setNativeCode(s, "startswith", strStartsWith),
 		"endswith":     setNativeCode(s, "endswith", strEndsWith),
+		"ljust":        setNativeCode(s, "ljust", strLJust),
+		"rjust":        setNativeCode(s, "rjust", strRJust),
 		"lstrip":       setNativeCode(s, "lstrip", strLStrip),
 		"rstrip":       setNativeCode(s, "rstrip", strRStrip),
 		"removeprefix": setNativeCode(s, "removeprefix", strRemovePrefix),
@@ -542,6 +545,35 @@ func strEndsWith(s *scope, args []pyObject) pyObject {
 	return newPyBool(strings.HasSuffix(string(self), string(x)))
 }
 
+func strLJust(s *scope, args []pyObject) pyObject {
+	return strJust(s, args, true)
+}
+
+func strRJust(s *scope, args []pyObject) pyObject {
+	return strJust(s, args, false)
+}
+
+func strJust(s *scope, args []pyObject, left bool) pyObject {
+	self := string(args[0].(pyString))
+	width := int(args[1].(pyInt))
+	fillchar := string(args[2].(pyString))
+
+	// utf8.RuneCountInString rather than objLen, because objLen returns the length of a string in
+	// bytes rather than characters:
+	s.Assert(utf8.RuneCountInString(fillchar) == 1, "fillchar must be exactly one character long")
+
+	count := width - utf8.RuneCountInString(self)
+	// strings.Repeat's second operand must be non-negative, otherwise it panics. In that case, no
+	// changes need to be made to the input string anyway, so just return a copy of it.
+	if count <= 0 {
+		return pyString(self)
+	}
+	if left {
+		return pyString(strings.Repeat(fillchar, count) + self)
+	}
+	return pyString(self + strings.Repeat(fillchar, count))
+}
+
 func strLStrip(s *scope, args []pyObject) pyObject {
 	self := args[0].(pyString)
 	cutset := args[1].(pyString)

src/parse/asp/interpreter_test.go

@@ -730,3 +730,43 @@ func TestBreakLoop(t *testing.T) {
 	assert.NoError(t, err)
 	assert.EqualValues(t, pyString("ok"), s.Lookup("x"))
 }
+
+func TestStrLjust(t *testing.T) {
+	s, err := parseFile("src/parse/asp/test_data/interpreter/str/ljust.build")
+	assert.NoError(t, err)
+	assert.EqualValues(t, pyString("   kittens"), s.Lookup("s1"))
+	assert.EqualValues(t, pyString("+++kittens"), s.Lookup("s2"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s3"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s4"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s5"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s6"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s7"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s8"))
+	assert.EqualValues(t, pyString("£££kittens"), s.Lookup("s9"))
+	assert.EqualValues(t, pyString("++++££kittens££"), s.Lookup("s10"))
+	// Make sure the original string wasn't modified:
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("orig"))
+
+	_, err = parseFile("src/parse/asp/test_data/interpreter/str/ljust_multiple_fillchars.build")
+	assert.Error(t, err, "fillchar must be exactly one character long")
+}
+
+func TestStrRjust(t *testing.T) {
+	s, err := parseFile("src/parse/asp/test_data/interpreter/str/rjust.build")
+	assert.NoError(t, err)
+	assert.EqualValues(t, pyString("kittens   "), s.Lookup("s1"))
+	assert.EqualValues(t, pyString("kittens+++"), s.Lookup("s2"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s3"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s4"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s5"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s6"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s7"))
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("s8"))
+	assert.EqualValues(t, pyString("kittens£££"), s.Lookup("s9"))
+	assert.EqualValues(t, pyString("££kittens££++++"), s.Lookup("s10"))
+	// Make sure the original string wasn't modified:
+	assert.EqualValues(t, pyString("kittens"), s.Lookup("orig"))
+
+	_, err = parseFile("src/parse/asp/test_data/interpreter/str/rjust_multiple_fillchars.build")
+	assert.Error(t, err, "fillchar must be exactly one character long")
+}

src/parse/asp/test_data/interpreter/str/ljust.build

@@ -0,0 +1,16 @@
+orig = "kittens"
+
+s1 = orig.ljust(10)
+s2 = orig.ljust(10, "+")
+s3 = orig.ljust(len(orig))
+s4 = orig.ljust(len(orig), "+")
+s5 = orig.ljust(0)
+s6 = orig.ljust(0, "+")
+s7 = orig.ljust(-5)
+s8 = orig.ljust(-5, "+")
+
+# 1-character, 2-byte fillchar:
+s9 = orig.ljust(10, "£")
+
+# Input string containing a multi-byte character:
+s10 = "££kittens££".ljust(15, "+")

src/parse/asp/test_data/interpreter/str/ljust_multiple_fillchars.build

@@ -0,0 +1 @@
+s = "kittens".ljust(10, "!!")

src/parse/asp/test_data/interpreter/str/rjust.build

@@ -0,0 +1,16 @@
+orig = "kittens"
+
+s1 = orig.rjust(10)
+s2 = orig.rjust(10, "+")
+s3 = orig.rjust(len(orig))
+s4 = orig.rjust(len(orig), "+")
+s5 = orig.rjust(0)
+s6 = orig.rjust(0, "+")
+s7 = orig.rjust(-5)
+s8 = orig.rjust(-5, "+")
+
+# 1-character, 2-byte fillchar:
+s9 = orig.rjust(10, "£")
+
+# Input string containing a multi-byte character:
+s10 = "££kittens££".rjust(15, "+")

src/parse/asp/test_data/interpreter/str/rjust_multiple_fillchars.build

@@ -0,0 +1 @@
+s = "kittens".rjust(10, "!!")