[New Feature] Add padStart, padEnd, padZero langlib functions to string.bal

[SandaruJayawardana]

User can easily add padding in strings with this feature.

• padStart - Adds padding to the start of a string

# Adds padding to the start of a string.
# Adds sufficient `padChar` characters at the start of `str` to make its length be `len`.
# If the length of `str` is >= `len`, returns `str`.
# 
# + str - the string to pad
# + len - the length of the string to be returned
# + padChar - the character to use for padding `str`; defaults to a space character
# + return - `str` padded with `padChar` 
public isolated function padStart(string str, int len, Char padChar = " ") returns string = external;

Ex:

str - "Ballerina"
len - 12
padChar - "-"
return - "---Ballerina"
Explanation: Length of the given string is 9. Target length is 12. Since 12 - 9 = 3 (> 0), we have to add 3 of - as the padding.

str - "Ballerina"
len - 9
padChar - "-"
return - "Ballerina"
Explanation: Length of the given string is 9. Target length is 9. Since 9 - 9 = 0 (<= 0), we don't need to add any padding.

• padEnd - Adds padding to the end of a string

# Adds padding to the end of a string.
# Adds sufficient `padChar` characters to the end of `str` to make its length be `len`.
# If the length of `str` is >= `len`, returns `str`.
# 
# + str - the string to pad
# + len - the length of the string to be returned
# + padChar - the character to use for padding `str`; defaults to a space character
# + return - `str` padded with `padChar` 
public isolated function padEnd(string str, int len, Char padChar = " ") returns string = external;

Ex:

str - "Ballerina"
len - 12
padChar - "-"
return - "Ballerina---"
Explanation: Length of the given string is 9. Target length is 12. Since 12 - 9 = 3 (> 0), we have to add 3 of - as the padding.

str - "Ballerina"
len - 5
padChar - "-"
return - "Ballerina"
Explanation: Length of the given string is 9. Target length is 5. Since 5 - 9 = -4 (<= 0), we don't need to add any padding.

• padZero - Adds padding to the start of a string

# Pads a string with zeros.
# The zeros are added at the start of the string, after a `+` or `-` sign if there is one.
# Sufficient zero characters are added to `str` to make its length be `len`.
# If the length of `str` is >= `len`, returns `str`.
# 
# + str - the string to pad
# + len - the length of the string to be returned
# + zeroChar - the character to use for the zero; defaults to ASCII zero `0`
# + return - `str` padded with zeros 
public isolated function padZero(string str, int len, Char zeroChar = "0") returns string = external;

Ex:

str - "100"
len - 5
zeroChar -
return - "00100"
Explanation: Length of the given string is 3. Target length is 5. Since 5 - 3 = 2 (> 0), we have to add 2 of 0 as the padding.

str - "-100"
len - 5
padChar -
return - "-0100"
Explanation: Length of the given string is 4. Target length is 5. Since 5 - 4 = 1 (> 0), we have to add 1 of 0 as the padding.

Your input is much appreciated on this.

Lang issues: #35546, #35548

[SandaruJayawardana]

Since maximum length of Java String is 2^31 - 1, we have to give an error when the given len is greater than 2^31 - 1. It is an edge case. Is there any size limitation for ballerina string? otherwise, we have to give an error like not yet supported. right?

[gimantha]

Since maximum length of Java String is 2^31 - 1, we have to give an error when the given len is greater than 2^31 - 1. It is an edge case. Is there any size limitation for ballerina string? otherwise, we have to give an error like not yet supported. right?

I think runtime error would work. This is a very edge case and implementation specific error. As per the spec I don't see any limitation for ballerina strings

[KavinduZoysa]

Please check #35598 (comment)

[KavinduZoysa]

@gimantha, @SandaruJayawardana, I saw that the implementation is done in java. Can't we implement it using ballerina?

[SandaruJayawardana]

Since we have used java string to keep ballerina string in jballerina, it won't work even we implement it using ballerina.