Release Notes for EPANET 2.2 (Draft)
============================
This document describes the changes and updates that have been made in version 2.2 of EPANET.

## Thread-Safe API Functions

A duplicate set of API functions has been provided that allow multiple EPANET projects to be analyzed concurrently in a thread-safe manner. These functions maintain the same name as the original but use a `EN_` prefix instead of `EN`. In addition, the first argument to each of these functions is a handle that identifies the particular project being analyzed. For example, instead of writing:

`ENgetnodevalue(nodeIndex, EN_ELEVATION, &elev)`

one would use:

`EN_getnodevalue(ph, nodeIndex, EN_ELEVATION, &elev)`

where `ph` is the handle assigned to the project. 

Two new functions have been added to the API to manage the creation and deletion of project handles. `EN_createproject` creates a new project along with its handle, while `EN_deleteproject` deletes a project. An example of using the thread-safe version of the API is shown below:
```
#include "epanet2_2.h"
int runEpanet(char *finp, char *frpt)
{
    EN_Project ph = 0;
    int err;
    err = EN_createproject(&ph);
    if (err) return err;
    err = EN_open(ph, finp, frpt, "");
    if (!err) err = EN_solveH(ph);
    if (!err) err = EN_report(ph);
    EN_close(ph);
    EN_deleteproject(ph);
    return err;
}
```
Prototypes of the thread-safe functions appear in the `epanet2_2.h` header file while `epanet2.h` contains prototypes of the legacy-style API functions. The enumerated constants used with both types of functions have been moved to `epanet2_enums.h`.

## Network Building By Code

API users now have the ability to build a complete EPANET network model using just function calls, without the need to open an EPANET-formatted input file. All types of network objects can be created and have their properties set using these calls, including both simple and rule-based controls.  Here is an example of building a simple 2-node, 1-pipe network just through code:
```
#include "epanet2_2.h"
int buildandrunEpanet(char *rptfile)
{
    // Create and initialize a project using gpm for flow
    // units and the Hazen-Williams formula for head loss
    EN_Project ph = 0;
    int err, index;
    err = EN_createproject(&ph);
    if (err) return err;
    EN_init(ph, rptfile, "", EN_GPM, EN_HW);
    
    //Add a junction node with 710 ft elevation and 500 gpm demand
    EN_addnode(ph, "J1", EN_JUNCTION, &index);
    EN_setjuncdata(ph, index, 710, 500, "");
    
    // Add a reservoir node at 800 ft elevation
    EN_addnode(ph, "R1", EN_RESERVOIR, &index);
    EN_setnodevalue(ph, index, EN_ELEVATION, 800);
    
    // Add a 5280 ft long, 14-inch pipe with C-factor of 100
    // from the reservoir to the demand node
    EN_addlink(ph, "P1", EN_PIPE, "R1", "J1", &index);
    EN_setpipedata(ph, index, 5280, 14, 100, 0);
    
    // Solve for hydraulics and report nodal results
    EN_setreport(ph, "NODES ALL");
    err = EN_solveH(ph);
    if (!err) err = EN_report(ph);
    
    // Close and delete the project
    EN_close(ph);
    EN_deleteproject(ph);
    return err;
}
```
Instead of using `EN_open` to load data from a file, `EN_init` is used to initialize a project with the names of its report and binary files, and its flow units and head loss formula. The legacy-style API has a complementary set of functions for building networks from code. 

## Additional Convergence Parameters

Two new analysis options have been added to provide more rigorous convergence criteria for EPANET's hydraulic solver. In the API they are named `EN_HEADERROR` and `EN_FLOWCHANGE` while in the `[OPTIONS]` section of an EPANET input file they are named `HEADERROR` and `FLOWCHANGE`, respectively.

`EN_HEADERROR` is the maximum head loss error that any network link can have for hydraulic convergence to occur. A link's head loss error is the difference between the head loss found as a function of computed flow in the link (such as by the Hazen-Williams equation for a pipe) and the difference in computed heads for the link's end nodes. The units of this parameter are feet (or meters for SI units). The default value of 0 indicates that no head error limit applies.

`EN_FLOWCHANGE` is the largest change in flow that any network element (link, emitter, or pressure-dependent demand) can have for hydraulic convergence to occur. It is specified in whatever flow units the project is using. The default value of 0 indicates that no flow change limit applies.

These new parameters augment the current `EN_ACCURACY` option which always remains in effect. In addition, both `EN_HEADERROR` and `EN_FLOWCHANGE` can be used as parameters in the `EN_getstatistic` (or `ENgetstatistic`) function to retrieve their computed values (even when their option values are 0) after a hydraulic solution has been completed.  

## More Efficient Node Re-ordering

EPANET's hydraulic solver requires solving a system of linear equations over a series of iterations until a set of convergence criteria are met. The coefficient matrix of this linear system is square and symmetric. It has a row for each network node and a non-zero off-diagonal coefficient for each link. The numerical effort needed to solve the linear system can be reduced if the nodes are re-ordered so that the non-zero coefficients cluster more tightly around the diagonal.

EPANET's original node re-ordering scheme has been replaced by the more efficient **Multiple Minimum Degree (MMD)** algorithm. On a series of eight networks ranging in size from 7,700 to 100,000 nodes MMD reduced the solution time for a single period (steady state) hydraulic analysis, where most of the work is for node-reordering, by an average of 55%. Since MMD did not reduce the time needed to solve the linear equations generated at each iteration of the hydraulic solver longer extended period simulations will not exhibit a similar speedup. 

## Improved Handling of Near-Zero Flows

EPANET's hydraulic solver can generate an ill-conditioned solution matrix when pipe flows approach zero unless some adjustment is made (i.e., as a pipe's flow approaches 0 its head loss gradient also approaches 0 causing its reciprocal, which is used to form the solution matrix's coefficients, to approach infinity). EPANET 2.0 used an arbitrary cutoff on head loss gradient to prevent it from becoming 0. This approach doesn't allow a pipe to follow any head loss v. flow relation in the region below the cutoff and can produce incorrect solutions for some networks (see [Estrada et al., 2009](https://ascelibrary.org/doi/full/10.1061/%28ASCE%29IR.1943-4774.0000100)).

The hydraulic solver has been modified to use a linear head loss v. flow relation for flows approaching zero. For the Darcy-Weisbach equation, the linear Hagen-Poiseuille formula is used for laminar flow where the Reynolds Number is <= 2000. For the Hazen-Williams and Chezy-Manning equations, if the head loss gradient at a given flow is below the EPANET 2.0 gradient cutoff then a linear head loss relation is used whose slope equals the cutoff. EPANET 2.2 is now able to correctly solve the examples presented in Estrada et al. (2009) as well as those in [Gorev et al., (2013)](https://ascelibrary.org/doi/10.1061/%28ASCE%29HY.1943-7900.0000694) and [Elhay and Simpson (2011)](https://ascelibrary.org/doi/10.1061/%28ASCE%29HY.1943-7900.0000411).

## Pressure Dependent Demands

EPANET has always employed a Demand Driven Analysis (**DDA**) when modeling network hydraulics. Under this approach nodal demands at a given point in time are fixed values that must be delivered no matter what nodal heads and link flows are produced by a hydraulic solution. This can result in situations where required demands are satisfied at nodes that have negative pressures - a physical impossibility. 

To address this issue EPANET has been extended to use a Pressure Driven Analysis (**PDA**) if so desired. Under **PDA**, the demand D delivered at a node depends on the node's available pressure P according to:

D = Dfull * [ (P - Pmin) / (Preq - Pmin) ]^Pexp

where Dfull is the full demand required, Pmin is the pressure below which demand is zero, Preq is the pressure required to deliver the full required demand and Pexp is an exponent. When P < Pmin demand is 0 and when P > Preq demand equals Dfull.

To implement pressure driven analysis four new parameters have been added to the [OPTIONS] section of the EPANET input file:

| Parameter | Description  | Default |
|-----------|--------------|---------|
| `DEMAND MODEL` | either DDA or PDA | DDA |
| `MINIMUM PRESSURE` | value for Pmin | 0
| `REQUIRED PRESSURE` | value for Preq | 0.1
| `PRESSURE EXPONENT` | value for Pexp | 0.5 |

These parameters can also be set and retrieved in code using the following API functions
```
int ENsetdemandmodel(int modelType, double pMin, double pReq, double pExp);
int ENgetdemandmodel(int *modelType, double *pMin, double *pReq, double *pExp);
```
for the legacy API and
```
int EN_setdemandmodel(EN_Project ph, int modelType, double pMin, double pReq, double pExp);
int EN_getdemandmodel(EN_Project ph, int *modelType, double *pMin, double *pReq, double *pExp);
```
for the thread-safe API. Some additional points regarding the new **PDA** option are:
 - If no DEMAND  MODEL and its parameters are specified then the analysis defaults to being demand driven (**DDA**).
 - This implementation of **PDA** assumes that the same parameters apply to all nodes in the network. Extending the framework to allow different parameters for specific nodes is left as a future feature to implement.
 - Preq must be at least 0.1 (either psi or m) higher than Pmin to avoid numerical issues caused by having too steep a demand curve.
 - Using `EN_DEFICIENTNODES` as the argument to `EN_getstatistic` (or `ENgetstatistic`) will retrieve the number of nodes that are pressure deficient. These are nodes with positive required demand whose pressure is below 0 under **DDA**  or below Preq under **PDA**.
 - Using `EN_DEMANDREDUCTION` as an argument will retrieve the total percent reduction of demands at pressure deficient nodes under **PDA**.
 - Using `EN_DEMANDDEFICIT` with the `EN_getnodevalue` (or `ENgetnodevalue`) function will return the amount of demand reduction produced by a **PDA** at any particular node.

## Tank Overflows
EPANET has always prevented tanks from overflowing by closing any links that supply inflow to a full tank. A new option `EN_CANOVERFLOW`, has been added to the list of Tank node properties. When set to 1 it will allow its tank to overflow when it becomes full. The spillage rate is returned in the tank's EN_DEMAND property. The default value for `EN_CANOVERFLOW` is 0 indicating that the tank cannot overflow.

For the input file, a new field has been appended to the data supplied for each tank in the `[TANKS]` section of the file. A value of **YES** indicates that the tank can over flow while **NO** (the default) indicates that it cannot. For the volume curve field that precedes it, an asterisk (*) can be used if the tank does not utilize a volume curve.

## Improved Water Quality Mass Balance

As described by [Davis et al. (2018)](https://www.drink-water-eng-sci.net/11/25/2018/dwes-11-25-2018.pdf) EPANET's water quality simulations can result in some significant mass balance errors when modeling short term mass injections (errors are much smaller for continuous source flows). The entire water quality engine has been re-written to eliminate these errors. It still uses the Lagrangian Time Driven transport method but now analyzes each network node in topologically sorted order rather than in arbitrary order.

A Mass Balance Report now appears the end of a simulation's Status Report that lists the various components (inflow, outflow, reaction) that comprise the network's overall mass balance. In addition `EN_MASSBALANCE` can be used as a parameter in the `EN_getstatistic` (or `ENgetstatistic`) function to retrieve the Mass Balance Ratio (Total Outflow Mass / Total Inflow Mass) at any point during a water quality simulation.

With this change EPANET 2.2 now produces perfect mass balances when tested against the networks used in Davis et al. (2018).

## New API functions
|Function|Description|
|--------|-----------|
|`EN_createproject` | Creates a new EPANET project |
|`EN_deleteproject` | Deletes an EPANET project |
|`EN_init`|Initializes an EPANET project|
|`EN_setflowunits`|Sets the project's flow units|
|`EN_addnode`|Adds a new node to a project|
|`EN_addlink`|Adds a new link to a project|
|`EN_addcontrol`|Adds a new simple control to a project|
|`EN_addrule`|Adds a new control rule to a project|
|`EN_deletenode`|Deletes a node from the project|
|`EN_deletelink`|Deletes a link from the project|
|`EN_deletepattern`|Deletes a time pattern from the project|
|`EN_deletecurve`|Deletes a data curve from the project|
|`EN_deletecontrol`|Deletes a simple control from the project|
|`EN_deleterule`|Deletes a rule-based control from the project|
|`EN_setnodeid`|Changes the ID name for a node|
|`EN_setjuncdata` |Sets values for a junction's parameters |
|`EN_settankdata` |Sets values for a tank's parameters| 
|`EN_setlinkid`|Changes the ID name for a link|
|`EN_setlinknodes`|Sets a link's start- and end-nodes|
|`EN_setlinktype`|Changes the type of a specific link|
|`EN_setpipedata`|Sets values for a pipe's parameters|
|`EN_getdemandmodel`|Retrieves the type of demand model in use |
|`EN_setdemandmodel`|Sets the type of demand model to use|
|`EN_adddemand`|Adds a new demand category to a node|
|`EN_deletedemand`|Deletes a demand category from a node|
|`EN_getdemandindex`|Finds a demand category's index given its name|
|`EN_getdemandname`|Gets the name of a node's demand category|
|`EN_setdemandname`|Sets the name of a node's demand category|
|`EN_setdemandpattern`|Assigns a time pattern to a node's demand category |
|`EN_setpatternid`|Changes the ID name of a time pattern|
|`EN_setcurveid`|Changes the ID name of a data curve|
|`EN_getcurvetype`|Gets a curve's type|
|`EN_setheadcurveindex`|Sets the index of a head curve used by a pump |
|`EN_getruleinfo`|Gets the number of elements in a rule-based control |
|`EN_getruleid` | Gets the name assigned to a rule-based control |
|`EN_getpremise`|Gets the contents of a premise in a rule-based control|
|`EN_setpremise`|Sets the contents of a premise in a rule-based control|
|`EN_setpremiseindex`|Sets the index of an object in a premise of a rule-based control|
|`EN_setpremisestatus`|Sets the status of an object in a premise of a rule-based control|
|`EN_setpremisevalue`|Sets the value of a property in a premise of a rule-based control|
|`EN_getthenaction`|Gets the contents of a THEN action in a rule-based control|
|`EN_setthenaction`|Sets the contents of a THEN action in a rule-based control|
|`EN_getelseaction`|Gets the contents of an ELSE action in a rule-based control|
|`EN_setelseaction`|Sets the contents of an ELSE action in a rule-based control|
|`EN_setrulepriority`|Sets the priority of a rule-based control|
|`EN_gettitle` |Gets a project's title |
|`EN_settitle` |Sets a project's title |
|`EN_getcomment` |Gets the descriptive comment assigned to an object|
|`EN_setcomment` |Assigns a descriptive comment to an object|
|`EN_clearreport` |Clears the contents of a project's report file |
|`EN_copyreport` | Copies the contents of a project's report file |
|`EN_getresultindex` | Gets the order in which a node or link was saved to file |
|`EN_getvertexcount` | Gets the number of vertex points in a link |
|`EN_getvertex` | Gets the coordinates of a vertex point in a link |
|`EN_setvertices` | Assigns a new set of vertex points to a link |
In addition to these new functions, a tank's volume curve `EN_VOLCURVE` can be set using `EN_setnodevalue` and `EN_setlinkvalue` can now be used to set the following pump properties:
 - `EN_PUMP_POWER` (constant power rating)
 - `EN_PUMP_HCURVE` (head characteristic curve)
 - `EN_PUMP_ECURVE` (efficiency curve)
 - `EN_PUMP_ECOST` (average energy price)
 - `EN_PUMP_EPAT` (energy pricing pattern)
 - `EN_LINKPATTERN` (speed setting pattern)
 
Access to the following global energy options have been added to  `EN_getoption` and `EN_setoption`:
 - `EN_GLOBALEFFIC` (global pump efficiency)
 - `EN_GLOBALPRICE` (global average energy price per kW-Hour)
 - `EN_GLOBALPATTERN`	(global energy price pattern)
 - `EN_DEMANDCHARGE` (price per maximum kW of energy consumption)

## New API Constants
### Node value types:
- `EN_CANOVERFLOW`
- `EN_DEMANDDEFICIT`

### Link value types:
- `EN_PUMP_STATE`
- `EN_PUMP_EFFIC`
- `EN_PUMP_POWER`
- `EN_PUMP_HCURVE`
- `EN_PUMP_ECURVE`
- `EN_PUMP_ECOST`
- `EN_PUMP_EPAT`

### Count types:
 - `EN_RULECOUNT`

### Head loss formula:
 - `EN_HW`
 - `EN_DW`
 - `EN_CM`

### Hydraulic option types:
 - `EN_HEADERROR`
 - `EN_FLOWCHANGE`
 - `EN_HEADLOSSFORM`
 - `EN_GLOBALEFFIC`
 - `EN_GLOBALPRICE`
 - `EN_GLOBALPATTERN`
 - `EN_DEMANDCHARGE`
 - `EN_SP_GRAVITY`
 - `EN_SP_VISCOS`
 - `EN_EXTRA_ITER`
 - `EN_CHECKFREQ`
 - `EN_MAXCHECK`
 - `EN_DAMPLIMIT`

### Quality option types:
- `EN_SP_DIFFUS`
 - `EN_BULKORDER`
 - `EN_WALLORDER`
 - `EN_TANKORDER`
 - `EN_CONCENLIMIT`
 
### Simulation statistic types:
 - `EN_MAXHEADERROR`
 - `EN_MAXFLOWCHANGE`
 - `EN_MASSBALANCE`
 - `EN_DEFICIENTNODES`
 - `EN_DEMANDREDUCTION`

### Action code types:
 - `EN_UNCONDITIONAL`
 - `EN_CONDITIONAL`

### Curve types:
 - `EN_VOLUME_CURVE`
 - `EN_PUMP_CURVE`
 - `EN_EFFIC_CURVE`
 - `EN_HLOSS_CURVE`
 - `EN_GENERIC_CURVE`

### Demand model types:
 - `EN_DDA`
 - `EN_PDA`

## Documentation
Doxygen files have been created to generate a complete Users Guide for version 2.2's API. The guide's format is similar to the original EPANET Programmer's Toolkit help file and can be produced as a set of HTML pages, a Windows help file or a PDF document. 
  
## Authors contributing to this release:
 - List item
