## String class using S4 methods
setClass("String",representation(string = "character"))

## Simple interface to make a new instance
string = function(x) {
  new("String",string=as.character(x))
}

### length,summary and show are all different
### * length is a "primitive"
### * summary is a S3 method
### * show is a S4 standardGeneric
### They all get treated the same here. No need to define them as as
### generic as done below. These need to have their arguments match
### (or possibly extend) those of the generic function. See ?setMethod
### for the gory details.

## length
setMethod("length","String",function(x) nchar(x@string))

setMethod("summary","String",
          function(object,...) {
            return(table(strsplit(object@string,""))) 
          })
## print method
setMethod("show","String",
         function(object) {
           cat(object@string,"\n")
         })

### Slice is not defined. It is made a generic object first, with the
### default definition setting up the formal arguments and default
### values. This is also done with the other remaining function
### definitions.

setGeneric("slice", function(object,index) standardGeneric("slice"))
setMethod("slice","String",
          function(object,index) {
            tmp = paste(unlist(strsplit(object@string,""))[index],
              sep="",collapse="")
            return(string(tmp))
          })

setGeneric("concat", function(e1,e2) standardGeneric("concat"))
setMethod("concat","String",
          function(e1,e2) {
            tmp = paste(e1@string,e2,sep="",collapse="")
            return(string(tmp))
          })


setGeneric("rept", function(e1,e2) standardGeneric("rept"))
setMethod("rept","String",
          function(e1,e2) {
            tmp = paste(rep(e1@string,e2),sep="",collapse="")
            return(string(tmp))
          })

### return vector of character type. Can't return vector of "String"
### type only a list
setMethod("split","String",
          function(x,f) {
            tmp =   unlist(strsplit(x@string,f))
            return(tmp)
          })

setGeneric("upcase", function(object) standardGeneric("upcase"))
setMethod("upcase","String",function(object) {
  string(toupper(object@string))
})

setGeneric("downcase", function(object) standardGeneric("downcase"))
setMethod("downcase","String",function(object) {
  string(tolower(object@string))
})

### These are syntactical shortcuts to some of the functions above The
### setMethod works for them. They need to have their arguments match
### those of the generic functions. This is why "[" has such a funny
### looking argument list, as the generic function "[" is used for
### vectors, matrices and arrays.
setMethod("[","String",function(x,i,j,...,drop) slice(x,i))
setMethod("+","String",function(e1,e2) concat(e1,e2))
setMethod("*","String",function(e1,e2) rept(e1,e2))
setMethod("/","String",function(e1,e2) split(e1,e2))


### A Sentence class extending String
setClass("Sentence",contains=c("String"))

## a new instance
sentence = function(x) {
  new("Sentence",string=x)
}

## a sentence method
setGeneric("punctuate", function(object) standardGeneric("punctuate"))
setMethod("punctuate","Sentence",function(object) {
  ## add a period, and capitalize the first letter
  chars = split(object,"")
  chars[1] = toupper(chars[1])
  if(chars[length(chars)] != ".")
    chars[length(chars)+1] = "."

  return(sentence(paste(chars,sep="",collapse="")))
})