public static String evaluate()

in logstash-core/src/main/java/org/logstash/StringInterpolation.java [66:137]
62 lines of code
18 McCabe index (conditional complexity)

    public static String evaluate(final Event event, final String template) throws JsonProcessingException {
        int open = template.indexOf("%{");
        int close = template.indexOf('}', open);
        if (open == -1 || close == -1) {
            return template;
        }
        final StringBuilder builder = STRING_BUILDER.get();
        int pos = 0;
        final int len = template.length();
        while (open > -1 && close > -1) {
            if (open > 0) {
                builder.append(template, pos, open);
            }
            if (template.regionMatches(open + 2, "+%s", 0, close - open - 2)) {
                // UNIX-style @timestamp formatter:
                // - `%{+%s}` -> 1234567890
                Timestamp t = event.getTimestamp();
                builder.append(t == null ? "" : t.toInstant().getEpochSecond());
            } else if (template.charAt(open+2) == '{' && (close < len) && template.charAt(close+1) == '}') {
                // JAVA-style @timestamp formatter:
                // - `%{{yyyy-MM-dd}}` -> `2021-08-11`
                // - `%{{YYYY-'W'ww}}` -> `2021-W32`
                // A special pattern to generate a fresh current time
                // - `%{{TIME_NOW}}` -> `2025-01-16T16:57:12.488955Z`
                close = close + 1; // consume extra closing squiggle
                final String pattern = template.substring(open+3, close-1);
                if (pattern.equals(TIME_NOW)) {
                    builder.append(new Timestamp());
                } else {
                    Optional.ofNullable(event.getTimestamp())
                            .map(Timestamp::toInstant)
                            .map(instant -> DateTimeFormatter.ofPattern(pattern)
                                    .withZone(ZoneOffset.UTC)
                                    .format(instant))
                            .ifPresent(builder::append);
                }
            } else if (template.charAt(open + 2) == '+') {
                // JODA-style @timestamp formatter:
                // - `%{+YYYY.MM.dd}` -> `2021-08-11`
                // - `%{+xxxx-'W'ww}  -> `2021-W32`
                final Timestamp t = event.getTimestamp();
                if (t != null) {
                    final String jodaTimeFormatPattern = template.substring(open + 3, close);
                    final org.joda.time.format.DateTimeFormatter jodaDateTimeFormatter = DateTimeFormat.forPattern(jodaTimeFormatPattern).withZone(DateTimeZone.UTC);
                    final DateTime jodaTimestamp = new DateTime(t.toInstant().toEpochMilli(), DateTimeZone.UTC);
                    final String formattedTimestamp = jodaTimestamp.toString(jodaDateTimeFormatter);
                    builder.append(formattedTimestamp);
                }
            } else {
                final String found = template.substring(open + 2, close);
                final Object value = event.getField(found);
                if (value != null) {
                    if (value instanceof List) {
                        builder.append(KeyNode.join((List) value, ","));
                    } else if (value instanceof Map) {
                        builder.append(ObjectMappers.JSON_MAPPER.writeValueAsString(value));
                    } else {
                        builder.append(value.toString());
                    }
                } else {
                    builder.append("%{").append(found).append('}');
                }
            }
            pos = close + 1;
            open = template.indexOf("%{", pos);
            close = template.indexOf('}', open);
        }
        if (pos < len) {
            builder.append(template, pos, len);
        }
        return builder.toString();
    }