Groundwork for multi codepage support

ebcdic.go

@@ -8,8 +8,18 @@ import (
 	"unicode/utf8"
 )
 
-// After careful consideration, I have decided that the code page we will
+// Implementations of Charset provide EBCDIC<->UTF-8 translation.
-// support for EBCDIC is IBM CP 1047.
+type Charset interface {
+	// Decode converts a slice of EBCDIC bytes into a UTF-8 string.
+	Decode(e []byte) string
+
+	// Encode converts a UTF-8 string into a slice of EBCDIC bytes.
+	Encode(s string) []byte
+}
+
+// After careful consideration, I have decided that the default code page we
+// will support for EBCDIC is IBM CP 1047. Other code pages may be globally
+// selected with the SetCodepage() function.
 //
 // In suite3270 (e.g. c3270/x3270), the default code page is what it calls
 // "brackets". This is CP37 with the [, ], Ý, and ¨ characters swapped around.
@@ -28,11 +38,49 @@ import (
 // In IBM PCOMM, CP37 is the default. For correct mapping of [, ], Ý, ¨, ^,
 // and ¬, you must switch the session parameters from "037 United States" to
 // "1047 United States".
+var currentCodepage Charset = Codepage1047()
+
+// SetCodepage sets the codepage/character set that go3270 uses. This is a
+// global setting, so if you're expecting clients to be configured to use a
+// character set other than go3270's default, cp1047, you should probably set
+// this during your application initialization and then leave it unchanged
+// after. This is _not_ a per-connection setting.
+func SetCodepage(cs Charset) {
+	currentCodepage = cs
+}
+
+// Internal implementation of the Charset interface we'll use for the codepage
+// support we provide.
+type charset struct {
+	// EBCDIC byte to Unicode code point for bytes 0x00-0xFF
+	e2u []rune
+
+	// Unicode code point to EBCDIC byte for codepoints 0x00-0xFF
+	u2e []byte
+
+	// Map of Unicode code points to EBCDIC bytes for codepoints >0xFF
+	highu2e map[rune]byte
+
+	// The EBCDIC substitute character to use if there is no EBCDIC character
+	// for the requested Unicode code point (typically 0x3F).
+	esub byte
+
+	// The "graphic escape" EBCDIC byte (is it ever anything other than 0x0E?)
+	ge byte
+
+	// Graphic escape codepage EBCDIC byte to Unicode code point for bytes
+	// 0x00-0xFF. Use rune '�' for unmapped bytes.
+	ge2u []rune
+
+	// Map of Unicode code points to graphic escape EBCDIC bytes.
+	u2ge map[rune]byte
+}
 
 // IBM CP 1047 <-> Unicode mappings from:
 // https://raw.githubusercontent.com/unicode-org/icu-data/refs/heads/main/charset/data/ucm/glibc-IBM1047-2.1.2.ucm
-
+func Codepage1047() Charset {
-var cp1047ToUnicode = []rune{
+	return &charset{
+		e2u: []rune{
 			/*         x0    x1    x2    x3    x4    x5    x6    x7    x8    x9    xA    xB    xC    xD    xE    xF */
 			/* 0x */ 0x00, 0x01, 0x02, 0x03, 0x9C, 0x09, 0x86, 0x7F, 0x97, 0x8D, 0x8E, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
 			/* 1x */ 0x10, 0x11, 0x12, 0x13, 0x9D, 0x85, 0x08, 0x87, 0x18, 0x19, 0x92, 0x8F, 0x1C, 0x1D, 0x1E, 0x1F,
@@ -50,9 +98,8 @@ var cp1047ToUnicode = []rune{
 			/* Dx */ 0x7D, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50, 0x51, 0x52, 0xB9, 0xFB, 0xFC, 0xF9, 0xFA, 0xFF,
 			/* Ex */ 0x5C, 0xF7, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5A, 0xB2, 0xD4, 0xD6, 0xD2, 0xD3, 0xD5,
 			/* Fx */ 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0xB3, 0xDB, 0xDC, 0xD9, 0xDA, 0x9F,
-}
+		},
-
+		u2e: []byte{
-var unicodeToCP1047 = []byte{
 			/*         x0    x1    x2    x3    x4    x5    x6    x7    x8    x9    xA    xB    xC    xD    xE    xF */
 			/* 0x */ 0x00, 0x01, 0x02, 0x03, 0x37, 0x2D, 0x2E, 0x2F, 0x16, 0x05, 0x25, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
 			/* 1x */ 0x10, 0x11, 0x12, 0x13, 0x3C, 0x3D, 0x32, 0x26, 0x18, 0x19, 0x3F, 0x27, 0x1C, 0x1D, 0x1E, 0x1F,
@@ -70,12 +117,22 @@ var unicodeToCP1047 = []byte{
 			/* Dx */ 0xAC, 0x69, 0xED, 0xEE, 0xEB, 0xEF, 0xEC, 0xBF, 0x80, 0xFD, 0xFE, 0xFB, 0xFC, 0xBA, 0xAE, 0x59,
 			/* Ex */ 0x44, 0x45, 0x42, 0x46, 0x43, 0x47, 0x9C, 0x48, 0x54, 0x51, 0x52, 0x53, 0x58, 0x55, 0x56, 0x57,
 			/* Fx */ 0x8C, 0x49, 0xCD, 0xCE, 0xCB, 0xCF, 0xCC, 0xE1, 0x70, 0xDD, 0xDE, 0xDB, 0xDC, 0x8D, 0x8E, 0xDF,
+		},
+		highu2e: map[rune]byte{},
+		esub:    0x3f,
+		ge:      0x08,
+		ge2u:    cp310ToUnicode,
+		u2ge:    unicodeToCP310,
+	}
 }
 
-// Furthermore, certain characters are supported in the "graphic escape"
+// Certain characters are supported in the "graphic escape" CP310. These are
-// CP310. These are arbitrary Unicode code points, so we will look them up via
+// arbitrary Unicode code points, so we will look them up via a map. For
-// a map. For simplicity of our mapping implementation, we will not support the
+// simplicity of our mapping implementation, we will not support the italic
-// italic underlined A-Z characters that require combining characters.
+// underlined A-Z characters that require combining characters.
+//
+// We will share this map among all of the codepages that we provide
+// implementations for.
 //
 // https://public.dhe.ibm.com/software/globalization/gcoc/attachments/CP00310.pdf
 var unicodeToCP310 = map[rune]byte{
@@ -127,25 +184,25 @@ var cp310ToUnicode = []rune{
 
 // decode will convert a CP1047 byte array into a UTF-8 Go string, handling
 // graphic escape to CP310.
-func decode(b []byte) string {
+func (cp *charset) Decode(b []byte) string {
 	runes := make([]rune, 0, len(b))
 	var escape bool
 	for i := range b {
 		if escape {
 			escape = false
-			if cp310ToUnicode[b[i]] != '�' {
+			if cp.ge2u[b[i]] != '�' {
-				runes = append(runes, cp310ToUnicode[b[i]])
+				runes = append(runes, cp.ge2u[b[i]])
 			} else {
 				runes = append(runes, 0x1A) // Unicode substitute
 			}
 		} else {
 			// Enter graphic escape mode if necessary.
-			if b[i] == 0x08 {
+			if b[i] == cp.ge {
 				escape = true
 				continue
 			}
 			// Otherwise perform the mapping.
-			runes = append(runes, cp1047ToUnicode[b[i]])
+			runes = append(runes, cp.e2u[b[i]])
 		}
 
 	}
@@ -153,7 +210,7 @@ func decode(b []byte) string {
 }
 
 // encode will convert a UTF-8 Go string into a CP1047 byte array.
-func encode(s string) []byte {
+func (cp *charset) Encode(s string) []byte {
 	out := make([]byte, 0, len(s))
 
 	for len(s) > 0 {
@@ -163,14 +220,14 @@ func encode(s string) []byte {
 			break
 		}
 
-		if int(r) < len(unicodeToCP1047) {
+		if int(r) < len(cp.u2e) {
-			out = append(out, unicodeToCP1047[r])
+			out = append(out, cp.u2e[r])
-		} else if v, ok := unicodeToCP310[r]; ok {
+		} else if v, ok := cp.u2ge[r]; ok {
 			// include graphic escape character to switch to CP310
-			out = append(out, 0x08, v)
+			out = append(out, cp.ge, v)
 		} else {
 			// replacement/substitute character
-			out = append(out, 0x3f)
+			out = append(out, cp.esub)
 		}
 		s = s[size:]
 	}

response.go

@@ -160,7 +160,7 @@ func readFields(c net.Conn, fm fieldmap, cols int) (map[string]string, error) {
 		if eor {
 			// Finish the current field
 			if infield {
-				value := decode(fieldval.Bytes())
+				value := currentCodepage.Decode(fieldval.Bytes())
 				debugf("Field %d: %s\n", fieldpos, value)
 				handleField(fieldpos, value, fm, values)
 			}
@@ -172,7 +172,7 @@ func readFields(c net.Conn, fm fieldmap, cols int) (map[string]string, error) {
 		if b == 0x11 {
 			// Finish the previous field, if necessary
 			if infield {
-				value := decode(fieldval.Bytes())
+				value := currentCodepage.Decode(fieldval.Bytes())
 				debugf("Field %d: %s\n", fieldpos, value)
 				handleField(fieldpos, value, fm, values)
 			}

screen.go

@@ -286,7 +286,7 @@ func showScreenInternal(screen Screen, values map[string]string,
 			}
 		}
 		if content != "" {
-			b.Write(encode(content))
+			b.Write(currentCodepage.Encode(content))
 		}
 
 		// If a writable field, add it to the field map. We add 1 to bufaddr