Let next read its HEX word from standard input

README.md

@@ -24,7 +24,7 @@ usage: fpbits decode FORMAT HEX...
        fpbits sum FORMAT NUMBER...
 ```
 
-Commands accept values as command-line arguments. For `decode`, `encode`, `convert`, and `sum`, a single `-` in place of the values reads whitespace-separated words from standard input. The input must be valid UTF-8; a leading byte-order mark is accepted.
+Commands accept values as command-line arguments. For `decode`, `encode`, `convert`, and `sum`, a single `-` in place of the values reads whitespace-separated words from standard input. `next` also accepts a single `-` in place of HEX; standard input must then hold exactly one word. The input must be valid UTF-8; a leading byte-order mark is accepted.
 
 Examples of commands include `fpbits decode f16 3c00`, `fpbits encode f16 0.1`, `fpbits convert f32 f16 3f800000`, `fpbits ulp f16 1`, `fpbits next --count 3 f16 3c00`, `fpbits range e4m3`, and `fpbits sum f16 2048 1 1`.
 

internal/cli/cmd_next.go

@@ -31,7 +31,18 @@ func RunNext(argv []string, stdin io.Reader) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	bits, err := fp.ParseHex(format, parsed.Args[1])
+	hex := parsed.Args[1]
+	if hex == "-" {
+		words, err := core.Words(stdin, parsed.Args[1:])
+		if err != nil {
+			return "", err
+		}
+		if len(words) != 1 {
+			return "", core.Failf("<stdin>: expected 1 word, got %d", len(words))
+		}
+		hex = words[0]
+	}
+	bits, err := fp.ParseHex(format, hex)
 	if err != nil {
 		return "", err
 	}

internal/cli/commands_test.go

@@ -1875,3 +1875,85 @@ error: 0e0
 		t.Errorf("sum with 401 significand digits returned (%d, %q, %q), want the complete literal diagnostic", status, stdout, stderr)
 	}
 }
+
+func TestNextReadsSingleHexWordFromStdin(t *testing.T) {
+	wantOutput := `offset  bits    value
+-1      0x3bff  9.995e-1
+0       0x3c00  1e0
++1      0x3c01  1.001e0
+`
+	tests := []struct {
+		name       string
+		args       []string
+		input      []byte
+		wantStatus int
+		wantStdout string
+		wantStderr string
+	}{
+		{
+			name:       "one word",
+			args:       []string{"next", "--count", "1", "f16", "-"},
+			input:      []byte("3c00\n"),
+			wantStatus: 0,
+			wantStdout: wantOutput,
+		},
+		{
+			name:       "byte-order mark, prefix, and CR LF",
+			args:       []string{"next", "--count", "1", "f16", "-"},
+			input:      []byte{0xef, 0xbb, 0xbf, '0', 'x', '3', 'c', '0', '0', '\r', '\n'},
+			wantStatus: 0,
+			wantStdout: wantOutput,
+		},
+		{
+			name:       "two words",
+			args:       []string{"next", "f16", "-"},
+			input:      []byte("3c00 3c01\n"),
+			wantStatus: 2,
+			wantStderr: "fpbits: <stdin>: expected 1 word, got 2\n",
+		},
+		{
+			name:       "no words",
+			args:       []string{"next", "f16", "-"},
+			input:      []byte(" \t\r\n"),
+			wantStatus: 2,
+			wantStderr: "fpbits: <stdin>: no input\n",
+		},
+		{
+			name:       "invalid UTF-8",
+			args:       []string{"next", "f16", "-"},
+			input:      []byte{0x33, 0xff},
+			wantStatus: 2,
+			wantStderr: "fpbits: <stdin>: input is not valid UTF-8\n",
+		},
+		{
+			name:       "bad hex word",
+			args:       []string{"next", "f16", "-"},
+			input:      []byte("3c0\n"),
+			wantStatus: 2,
+			wantStderr: "fpbits: invalid hex word \"3c0\" for f16 (expected 4 hex digits)\n",
+		},
+		{
+			name:       "unknown format is reported before stdin is read",
+			args:       []string{"next", "f17", "-"},
+			input:      []byte{0xff},
+			wantStatus: 2,
+			wantStderr: "fpbits: unknown format \"f17\" (expected f16, bf16, f32, f64, e5m2 or e4m3)\n",
+		},
+		{
+			name:       "too many arguments are reported before stdin is read",
+			args:       []string{"next", "f16", "-", "3c00"},
+			input:      []byte{0xff},
+			wantStatus: 2,
+			wantStderr: "fpbits: too many arguments\n",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			status, stdout, stderr, _, _ := runCLI(test.args, test.input)
+			if status != test.wantStatus || stdout != test.wantStdout || stderr != test.wantStderr {
+				t.Errorf("cli.Run(%q) = (%d, %q, %q), want (%d, %q, %q)", test.args, status, stdout, stderr, test.wantStatus, test.wantStdout, test.wantStderr)
+			}
+		})
+	}
+}